MASTERCARD ISSUE UNRESOLVED:
I strongly feel that you should look critically at your Checkout page. It seems that there is something wrong with your payment buttons. The CVV Code entry seems to be for VISA card only. Entries made in the box seem to be interpreted as that of VISA secure code.
I use my MasterCard to purchase products from other websites without any hitch, but your own checkout page seems to be totally weird. Do something about this urgently in order not to lose sales and customers alike.
I suggest that a member of your support staff should test the checkout with MasterCard and see what happens. This will help you to advice your customers appropriately.