A first payment that works in sandbox can hide what happens when the next charge fails.

Fail a renewal in your test environment and watch the result. Does access stay open, end at once, or follow a grace period you actually chose?

Providers usually retry the card and send failed-payment or past-due events. If those arrive and your handler returns 200 without changing anything, a customer may keep a paid feature for free, or lose it with no warning.

Check the notice you'd send them too. A silent change after a failed charge is how angry tickets start.