Sell software licence keys A key issued at checkout is the easy half. The half that costs you weekends is everything after: activation limits, validation from the app, and revoking a key when the payment behind it goes away.

Issuing a random string on a successful payment takes an afternoon. Then the real requirements arrive: how many machines may activate this key, what happens on a refund, what happens when a subscription lapses, how a customer sees their own keys without emailing you, and how your app checks a key without you running an uptime-critical service you did not plan for.

All of those questions are really the same question — the key has to stay tied to the payment that created it. If the key lives in one system and the payment in another, you end up reconciling them by hand.

• Issued on payment, against the order — not as a side effect you have to wire up.
• Activation limits per key, so a licence covers the number of machines you sold.
• Validated from your app over the API, with the key's current state as the answer.
• Revoked when the money goes away — refund or cancellation, without a manual step.
• Visible to the customer in the billing portal, so key retrieval is not a support ticket.
• Signed webhooks on issue, activation and revocation, so your own systems can react.

And because it is the same API as checkout and subscriptions, a perpetual licence, a subscription that carries a key, and a key with metered usage on top are the same shape of integration rather than three of them.