When using the test API, the transaction amount gives you the ability to test both successful and failed payments. When the transaction amount has zero cents or a number of cents other than the ones listed below, the test transaction will always be successful.
If you provide one of the following numbers for the number of cents you will receive a transaction failed response with the corresponding reason:
31 = Invalid Account
54 = Expired card - Card Only
51 = Declined
61 = Insufficient Funds
96 = Technical failure - Card Only
For more information around ‘transactions’, see our API documentation here.