Are you paying by PayPal? I had this problem and turned out that PayPal were being overzealous in their security department. They allowed the payment and even sent me a receipt, I got to the upload screen, tried to upload and was denied.
When I traced the problem it was due to PayPal flagging my account as a security problem and reimbursing the money back to my card!!
Once I had jumped through the PayPal security hoops my payment was allowed and I could upload.
Hope you get it sorted
