Polar billing integrations do not start at checkout. The first real question is usually simpler: Can my app create the customer and product records it will depend on later? If that part is shaky, checkout and subscription handling get harder to trust. You end up debugging billing logic when the actual bug was earlier: the customer ID did not persist, the product did not show up in the list call, or your app stored an internal user ID that never made it into the billing system. The boring part is the important part Polar's API has the objects you expect in a modern billing system: customers products checkouts orders subscriptions benefits webhooks The checkout flow is the part everyone thinks about. But checkout depends on earlier state. For example, a product has to exist before you can sell it. A customer has to map back to your own user model before you can reconcile access later.…