The part where mistakes cost money.
USE TEST MODE
Every gateway provides test credentials and test card numbers. Use them for all development.
Never develop against live keys. A test transaction against live keys is a real charge.
WHAT TO TEST
A successful payment A declined payment A payment abandoned midway A payment that succeeds at the gateway but where the callback fails A refund A duplicate submission
The failure paths matter more than the success path, because that is where money goes missing.
WEBHOOKS
Test that your endpoint receives and processes the notification. Verify the signature check works, and confirm an unsigned request is rejected.
Most gateways provide a delivery log showing attempts and responses.
BEFORE GOING LIVE
Switch to live keys, then make one real low-value purchase yourself and refund it. Test mode proves the integration; a live transaction proves the settlement and the emails.
AFTER GOING LIVE
Reconcile the first days of orders against the gateway dashboard. Discrepancies reveal callback problems early.
