When generating IDs for the Payment Batch API Call and the Payment Instructions API Call, I recommend starting with the ID 100,000 and counting upwards, since I've used some of the numbers below 100,000 to test. It seems that these IDs are discarded immediately after sending, but if we try to create 2 Batch Payments or 2 Payment Instructions with the same ID, the API throws an error. In any case, the API response to this call gives the real ID that has been assigned by the server, which we should 1) use in the other calls and 2) save in the database for reference.