Symptoms
OA Billing API methods can use transaction mechanism to execute several requests within one transaction (API Usage Basics documentation describes transaction mechanism usage in details).
Some API method is executed with AutoCommit=No and returns ID of database transaction. Another method joins that transaction but commits it when being completed.
Cause
Subsequent API methods were just attached to existing transaction but AutoCommit=No is missed.
Resolution
To prevent commitment of the following API methods that are executed in one transaction it is required to add parameter AutoCommit=No to each of them.