If you are selling a digital product, it may be advisable to use a third party payment processor like Clickbank or Plimus. They take care of all the payments, refunds and the disputes/chargebacks are also handled by them.
Few merchants seem to have a well designed system for error handling while processing payment. I had made a payment using Bill Desk, and since there was an error while processing the payment, I could not note down the transaction id. I then had to wait for nearly 36 hours before the payment could be processed properly and waste a lot of time following up.