Start with a written contract
An integration should define request fields, authentication, success and error responses, callbacks, retry behaviour and responsibility for each failure path.
Use idempotency for critical actions
Payments, payouts, orders and reward approvals should not be created twice because of a timeout or retry. A stable request key and server-side duplicate protection reduce this risk.
Store external and internal references
Keep your own operation ID together with the provider reference, beneficiary or customer reference, amount, status and timestamps. This makes support and reconciliation practical.
Treat callbacks as untrusted input
Verify signatures or credentials, validate fields, protect against replay and fetch final status from the provider where appropriate.
Design the exception path
Not every failure should be retried automatically. Some require user correction, provider review or manual operations. Good systems make that state visible instead of hiding it.