An integration succeeds when the business can rely on the result, not simply when one system can call another.
Agree which system owns the data.
Decide where a record is created, where it can change and how conflicting updates should be handled. A catalogue, customer or order record needs a clear source of truth.
Plan for exceptions.
Discuss missing fields, duplicate events, delayed updates and unavailable systems. Someone should know what happened, what needs attention and whether an operation can be retried.
Make the handover part of the scope.
Include monitoring, documentation and responsibility for ongoing changes in the delivery plan. A useful integration is something your team can understand and operate.