I'd also like to get to the bottom of this. I just bought that same Visa prepaid debit card and in the term and conditions it does not say you can't use it for international purchases, what it does say is that you can't use the card to make transactions in foreign currencies.
Also ttyler27, did you register the card? It says that in order to use the card over the phone or online you have to register it. Of course you can probably give any name/address when you do this, but it might make sense to register it with the address of the place you are going to have stuff shipped to, just for continuity. If this still doesn't work, let us know what you figure out. Maybe there are international gift cards, or maybe the seedbank you're ordering from will accept a transaction in US dollars. Is it Attitude you are ordering from?