Payments and Login First: What to Build in Your First Product

If you are building a first working product, build the login and the payment flow before anything else. They are the two parts that decide whether strangers can safely use the product and whether you can get paid for it. Every other feature sits on top of them, and both are painful to bolt on late.

I am Hanad, a senior engineer in Vienna. I build first real versions of products for founders at a fixed price, and almost every rescue job I take on has the same story: the screens looked fine, but logins or payments were the part that broke under real users. This guide explains what each piece must do, what to decide before any code is written, and how to test both before launch.

Key Takeaways

• Build identity and money first. Features that depend on knowing who the user is and whether they paid are the hardest to retrofit.
• Decide your billing model, and whether you use a payment processor or a Merchant of Record, before anyone writes integration code.
• Never trust the browser to tell you a payment succeeded. Fulfill from server-side webhook events, and make every handler safe to run twice.
• Use established, boring tools for login and payments rather than inventing your own, and test failures (declines, expired links, lost webhooks) as carefully as successes.

Table of Contents

• Why these two come before everything else
• What the login system actually has to do
• Decisions to make before any code
• How an online payment should flow
• Security and privacy checks that cannot wait
• Test both before real users arrive
• When logins or payments stop working
• My view: do less, but do these two properly
• What this costs with me
• FAQ
• Sources

Why these two come before everything else

A prototype can fake almost anything. A button can pretend to save, a dashboard can show sample data, and a pricing page can link to nowhere. Login and payments cannot be faked, because they are where your product meets the outside world.

Login answers one question: who is this person, and what are they allowed to see? Authentication verifies that a user is who they claim to be, and session management keeps that authenticated state alive between requests. OWASP's developer guide adds that authentication and access-control checks should protect every non-public resource, not just the pages you remembered.

Payments answer the second question: did this person pay, and what did they buy? If your data model does not connect users, plans and payments from the start, you end up migrating live customers later, which is exactly the kind of job founders then hire me to rescue.

There is also a practical reason. Once login and billing exist, every later feature has something solid to hang on: per-user data, free versus paid limits, an admin view of who is active. Without them, you build features twice.

What the login system actually has to do

A real login system is more than a form. At minimum it needs a database that stores credentials safely, secure password hashing, and session management. A practical guide to adding logins and payments to a first app lists the same three building blocks.

In plain terms, a first version needs these behaviors:

1. Sign-up that creates an account and sends a verification email.
2. Login that checks credentials and starts a session.
3. Logout that actually ends the session.
4. Password recovery that works without leaking information.
5. Access rules so one customer can never read another customer's data.

That last item is where most no-code and AI-generated prototypes fail. The screen hides the data, but the underlying query still returns it. If you are in that situation on Bubble, my five security checks before rebuilding show what to look for.

For a deeper walk-through of what to include and what to postpone, see what to build first for MVP login and auth.

Email verification and password reset

These two flows look trivial and are the most commonly broken. OWASP's email verification guidance says verification should use cryptographically secure, random, single-use, time-limited tokens, and that an account should not be enabled until the address is verified. Password reset follows the same pattern: single-use, time-limited tokens, consistent responses, and rate limits, without disclosing whether an email address exists.

That "consistent response" point matters more than it sounds. If your reset page says "no account with that email", anyone can use it to check who your customers are. The page should say the same thing either way: "If that address is registered, we sent a link."

The same OWASP guide adds a few more details worth putting in your spec:

• Send the temporary link only to the address registered on the account.
• Keep the expiry short.
• Store passwords as strong one-way salted hashes, never as plain text or reversible encryption.
• Notify the user after a reset, so a hijacked account does not go unnoticed.

Sessions, brute force and sensitive actions

Where the session lives matters. The OWASP Proactive Controls recommend server-side session management over client-side storage, because it allows universal logout and proper invalidation. In practice that means you can end every session for a user on every device if their account is compromised.

Other rules I build in by default, all from OWASP's digital identity checklist:

• Protect against brute-force attacks by locking accounts after a set number of failed attempts or adding a delay.
• Apply the same level of control to password reset and change as to account creation and login.
• Ask users to re-authenticate before critical operations such as changing a password or updating payment details.
• Prefer an established authentication service over a homegrown one, and offer multi-factor authentication for sensitive or high-value accounts.

One more rule, small but telling: login errors should not reveal which field was wrong. "Incorrect email or password" is the right message, not "password incorrect".

Build or buy the login?

For a first version, I almost always use a proven library or hosted identity service instead of writing authentication from scratch. It is less code to maintain and the next developer already knows it. The trade-off is cost and lock-in at scale, which is a real question once you sell to larger companies that want single sign-on. I cover that stage separately in fixed-price SSO for B2B SaaS. For a first product with a few hundred users, you rarely need it yet.

Hosted services also have limits you should read before choosing one. Auth0, for example, publishes rate limits on authentication endpoints, including one password-reset request per minute from an IP address to a given email on its Essentials and Professional plans. That is fine for real users, but it can surprise you during load tests or when your own support team tries to help a customer.

Decisions to make before any code

Billing is where founders lose the most time, and the expensive mistakes are made before development starts, not during it. Advice for solo founders from Freemius points to the same checklist: define the billing model, choose between a standard payment processor and a Merchant of Record, and plan tax compliance before integration code is written.

Here is how I frame it with clients.

1. How will you charge?

One-off purchase, monthly subscription, per-seat pricing, usage-based, or a paid pilot followed by a contract. Each one changes the data model: subscriptions need renewal and cancellation states, usage-based billing needs metering, per-seat needs team accounts. My guide to revenue model examples for B2B SaaS walks through the options. Pick the simplest model that you can explain in one sentence to a customer.

2. Processor or Merchant of Record?

With a standard payment processor, you are the seller. You collect and remit sales tax and VAT where required, and you handle disputes. With a Merchant of Record, a third party is the legal seller and takes on much of the tax work, usually for a higher fee. If you sell to businesses in a handful of countries, a processor plus a tax tool is common. If you sell low-priced digital products to consumers worldwide and have no finance staff, a Merchant of Record can save you real headaches. This is a business decision first and a technical one second, so make it with your accountant, not your developer.

3. What can users do before they pay?

Free trial, free tier, demo account, or nothing until payment? This decides whether you need to verify email before access, how you limit abuse, and what "not paid" looks like in your database. Write it down as a table of states: no account, account unverified, verified free, paid, payment failed, canceled.

4. Who is allowed to see what?

Single user per account, or teams with roles? Adding teams later is one of the most expensive changes to a codebase. If there is any chance you will sell to companies, plan an organization concept now, even if the first version only uses one user per organization.

How an online payment should flow

A payment flow has more moving parts than the checkout button suggests. With Stripe, the most common choice for first products, the pattern is well documented.

1. The customer clicks buy, and your server creates a checkout session or payment intent.
2. The customer pays on the hosted page or embedded form.
3. Stripe sends your server an event describing what happened.
4. Your server records the transaction and grants access.
5. The customer lands on a confirmation page.

The step that founders get wrong is step 4. Stripe states plainly that you cannot rely on your confirmation page alone to trigger fulfillment, because a customer can pay and then lose their connection before the page loads. Webhooks are required for subscriptions and for payment methods with delayed success notification. The redirect is for the customer's convenience, the webhook is for your records.

Webhooks, retries and refunds

Stripe's server-side integration guide recommends idempotency keys on all POST requests so a retried request does not create a second charge. Webhooks capture asynchronous events such as payment success, failure and cancellation, and the Refunds API handles full or partial refunds.

Stripe's webhook documentation adds the operational rules: the endpoint must be reachable over HTTPS, you must verify the event signature, and you should return a 2xx response quickly to avoid timeouts. Do the heavy work after acknowledging the event.

My own rule is that every webhook handler must be safe to run twice. Events can arrive more than once and out of order. If "grant access" runs twice, nothing bad should happen.

Subscriptions and failed payments

For subscriptions, the relevant events include creation, update and deletion of the subscription, invoice success or failure, and required customer authentication. Stripe's billing testing guide says to provision access when the invoice is paid and the subscription is active. On a failed payment, it suggests notifying the customer, enabling retries, or collecting new payment information.

Decide in advance what "failed" means for your product. Do users lose access immediately, after a grace period of several days, or do they keep read-only access? There is no universal answer, but the product must have one. A failed card is not a churned customer; it is usually an expired card, and a polite email recovers many of them.

Disputes are a separate matter. A customer who disputes a charge with their bank triggers a chargeback, and you have a limited window to respond with evidence. This walkthrough on responding to Stripe chargebacks shows the practical steps a small service business takes. Even for software, keep clear records of what was sold, when, and to whom.

If you want the subscription side done end to end, I do it as a fixed-price Stripe billing engagement for SaaS.

Security and privacy checks that cannot wait

Here is the short list I check on every project, whether I wrote the code or inherited it.

If you ship a mobile app, OWASP's mobile guidance adds two more: do not hardcode credentials, and use the platform's secure storage, such as Keychain on iOS or Keystore on Android. Not sure whether you need a native app at all? I compare the options in web app versus mobile app for SaaS.

Privacy belongs on this list too. Store only the personal data you need, and keep card details out of your own database entirely. If you serve European customers, a login system collects personal data from day one, so document what you store and why.

Test both before real users arrive

The happy path will work in a demo. Real users find the other paths. Stripe's guidance is to test successful and failed payments in the provider's test environment, exercise webhook delivery, and use test cards or test clocks to simulate failed renewals and time-dependent subscription changes.

My pre-launch checklist:

• Sign up with a new email, verify it, log out, log in.
• Use an expired verification link and an already used one. Both should fail cleanly.
• Request a password reset for an unknown email and for a real one. The screens should look identical.
• Pay with a successful test card, then a declined one, then one that requires extra authentication.
• Disable your webhook endpoint for a minute, make a payment, turn it back on, and confirm the event is retried and access is granted once.
• Advance a test clock past a renewal with a failing card and watch what your product does.
• Log in as customer A and try to open customer B's data by changing an ID in the URL.
• Refund a payment and confirm access changes as you intended.

If any of these produce a blank screen or a wrong state, you have found a launch bug instead of a customer complaint.

When logins or payments stop working

The usual causes are boring, which is good news, because boring problems are quick to diagnose.

• Payment succeeded but no access. The webhook did not arrive, failed signature verification, or timed out. Check the provider's event log first, then your server logs.
• Works in test, fails in live. Live and test mode use different API keys and different webhook secrets. This is the most common launch-day bug.
• Users cannot reset passwords. Email deliverability, an expired token, or a rate limit. Look at whether the email was sent before debugging anything else.
• Users see each other's data. An access rule was enforced in the interface but not in the query. Treat it as urgent.
• Duplicate charges or duplicate records. Missing idempotency keys or a webhook handler that is not safe to run twice.

When the prototype was generated by an AI builder or a no-code platform, these issues are tangled into the platform and hard to see. That is usually the point where a rebuild is cheaper than patching.

My view: do less, but do these two properly

Founders often ask me to start with the exciting feature and add login and payments "at the end". I push back on this, politely. The exciting feature is the part you can still change cheaply, and identity and money are not.

I keep scope small on purpose. One core workflow, one billing model, one login method, built on a stack that is deliberately boring: TypeScript, Next.js, Node, SQL and Stripe. The reason is that the next developer, whether that is you, a hire or someone years from now, should open the code and understand it in an afternoon. You own the code from the first commit, and you see working progress before you pay for the next step.

I also tell clients when they do not need custom software. If a hosted checkout link and a spreadsheet cover your first ten customers, start there. Custom login and billing earn their cost when real users, real money or an investor review are close.

— Hanad Kubat

What this costs with me

I work alone, so there is no agency overhead, no project-manager layer and no offshore markup in the price. A first real version with working login and payments starts from €12,000 at a fixed price, typically in two to four weeks. If you are unsure about scope or have an existing prototype, a €1,500 Prototype Audit takes three to five days and is credited against the build. Details are on my fixed-price app development page.

FAQ

Should I build login or payments first?

Login first, because payments need to attach to a user. Build them in the same sprint, though, and design the data model for both at once. Otherwise you will migrate users later to connect them to their payments.

Can I use Stripe without writing a webhook handler?

For a simple one-time purchase you can start with a payment link and confirm orders by hand. Once you sell subscriptions or want automatic access, Stripe's documentation says webhooks are required for reliable fulfillment. Build the handler before you have more customers than you can check manually.

Do I need a Merchant of Record?

It depends on where you sell and who handles tax. If you sell to consumers across many countries and have no finance help, it can remove a lot of work for a higher fee. If you sell business software in a few markets, a standard processor plus tax tooling is common. Talk to your accountant before choosing.

How long does it take to build a secure login and payment flow?

Inside a complete first product, I deliver both within a two to four week fixed-scope build. Security details such as verification tokens, rate limits and webhook signatures are part of the work, not extras.

My Lovable or Bubble prototype has login and Stripe already. Why rebuild?

Often you should not, until it hits a wall: it breaks under real users, fails a security review, or the platform bill grows. A short audit tells you which case you are in before you commit to a rebuild.

Sources

• Implement Digital Identity, OWASP Developer Guide
• Email Validation and Verification, OWASP Cheat Sheet Series
• C7: Secure Digital Identities, OWASP Top 10 Proactive Controls
• Mobile Application Security, OWASP Cheat Sheet Series
• Fulfill orders, Stripe Documentation
• Server-side integration, Stripe Documentation
• Test your Billing integration, Stripe Documentation
• Receive Stripe events in your webhook endpoint
• Essentials and Professional rate limits, Auth0 Docs
• Payment processing for SaaS, Freemius
• The Fastest Way to Add Logins, Data, and Payments to an App

Recommended

• Login and Auth for an MVP: What to Build First
• Startup product development steps: your guide to fast MVP launch
• Lean product creation guide: fast-track your SaaS MVP
• Product development best practices for non-technical founders