For your successful transaction/thank you message, perhaps explain the remainder of the shopping process to the customer, eg that they will expect a shipping notification within x number of days by email, and typical delivery dates etc. For failed transaction messages, you should provide details here about alternative ways the customer can pay you, about how they can phone you with their credit card number, explain that payment gateways are not always available 24/7. The more helpful information the customer has, the better chance this order will eventually be paid for and turned into a sale.
Successful transactions
Failed transations