UPDATE: With Stripe's help, we've resolved the problem. In the process, I've learned several things: - Do the numbers. In our case, we found that the decline

UPDATE: With Stripe's help, we've resolved the problem. In the process, I've learned several things: - Do the numbers. In our case, we found that the decline surge was caused purely by MasterCard payments in foreign currencies. - Actively encourage customers to call their bank to accept the payment. In our case, we modified the payment rejected page to give detailed instructions, including what information their bank would need. - The customer calls provide feedback to the bank's fraud detection system on a timescale of days. Our MasterCard rejections went from zero to 100% over three or four days, and then gradually back down to zero over three days. - The banks don't get IP addresses, so Appengine was not a factor in the declined payments. My only criticism of Stripe is that their declines help page describes declines as sporadic random events, which was not what we were experiencing. Knowing that decline surges happen - and the points above - would have hugely reduced the stress caused by the situation. But apart from that, Stripe are great. Their simple-but-powerful and awesomely-documented API is the best I've used, and the service itself just works. Thanks Stripe!