The prepaid vendor might have a policy against international purchases. It might be caused by either the fact that you are buying a Czech product, or buying a product through a US company.
Prepaid cards are dumb like that.
You will probably have to contact the card issuer to see why they are denying this transaction.
Otherwise, if you have an active PayPal account, you could *TRY* to link the card to your account, but from what I have read, this usually ends with you getting ripped off for a dollar each time you try.
PayPal might have something to say on this page
here, but it seems to be broken for the moment.
From what I understand, it can also be some sort of security protection on PayPal's side triggered by the lack of a name and address associated with the prepaid card.