In case if you still see pay button, you could probably try to use another card. I have Maestro and it was rejected, but my paying button disappeared and I was unable to try to pay again with another card.
So instead of the payment receipt I send them a letter explaining what happened with screen shots of the "Transaction Declined" window and they allowed me to pay with using a bank transfer. I uploaded that letter at the same place where they asked for a payment receipt, under the list of all your submitted documents.