Sign Up & Payments

User accounts and charging money. Only matters once you have validated demand.

Your app is on the internet. Now make it so people can create their own accounts, have their own data, and pay you money. This section is for builders who are ready to charge -- if you're still validating whether anyone wants this, start with Customer Discovery first.

User Accounts -- Their Own Space

Right now, everyone who visits your app sees the same thing. You need each person to have their own account -- their own login, their own data, their own experience.

What this gives you:

• People can sign up with their email (or sign in with Google -- one click)
• Each person sees only their own stuff
• They can log out and come back later -- their data is still there
• You know who your users are (email addresses, when they signed up)

What to tell your AI:

"Add user accounts using Supabase Auth. Let people sign up with email and password, or sign in with Google. Protect the main pages so only logged-in users can see them. Make sure each user can only see their own data."

Storing Each Person's Data

When someone creates an account and uses your app, their information needs to be saved somewhere -- a database. Think of it like spreadsheets in the cloud:

• A "users" spreadsheet -- one row per person. Their name, email, when they signed up, what they've paid for.
• A spreadsheet for your app's content -- whatever your users create. Projects, posts, assessments, orders -- depends on your idea.
• They connect to each other -- each piece of content belongs to one user. User A sees their stuff, User B sees theirs.

What to tell your AI: "Create a Supabase database for my app. I need to store [describe what your users create]. Set up row-level security so each user can only see their own data."

Accepting Payments

You want to charge money. Stripe handles the hard parts -- credit cards, subscriptions, receipts, taxes, refunds. You just connect it.

How it works:

1. A customer clicks "Buy" or "Upgrade" on your site
2. They're sent to a payment page that Stripe hosts (you never see their credit card)
3. They pay
4. Stripe notifies your app behind the scenes: "This person paid"
5. Your app upgrades their account

What to tell your AI: "Set up Stripe Checkout so I can charge $[amount] per month. When someone pays, update their plan in Supabase to 'paid'. Also handle cancellations -- when someone cancels, set them back to 'free'."

Sending Emails

At minimum, you'll want to send a welcome email when someone signs up. Later, you might add receipts, notifications, or weekly updates.

What to tell your AI: "Add Resend to send a welcome email when a new user signs up. Keep it simple -- just a thank-you with one sentence about what to do next."

Knowing When Things Break

Real users will find problems you never did. You need to know about them before your users email you.

• Sentry -- catches errors automatically and emails you. Tell your AI: "Add Sentry error tracking." Free to start.
• A simple check -- ask your AI: "Create a health check page at /api/health that returns 'ok'. If it ever stops working, the whole app is down."

Before You Share It -- Checklist

• Open your app in a private/incognito window. Can a brand new person sign up?
• Can they log in, log out, and log back in?
• Can they do the main thing your app is for?
• Does their data show up only for them (not for other users)?
• Can they pay? (Test with Stripe's test mode -- no real money moves)
• Do you get an email when someone signs up? When they pay?
• Does it work on a phone?
• Is there a loading indicator when things are loading? (No blank screens)

What This Actually Costs (as of mid-2026)

Every guide says "free to start." Here's what it actually costs as you grow:

At $19/month per customer and 100 paying users, you're making $1,900/mo with ~$1 in infrastructure costs. The margins are real. They stay real until you're big enough for it to be a good problem to have.