Hi,
I have exactly the same problem. I've always used virtual visa (MBNet) to buy lindens. I never had any problems using it in Second Life, and I shop online frequently in Amazon and others with this method.
I called the support line (both in the US and London) and submited a ticket to SL support. We've exchanged around 6 emails, and the Lindens keep saying that I should update my bill info (when I do that, my visa number is not validated). No alternatives are proposed. They ask me to make sure my bank allows charges from SL / LL - and I know there's no restrictions at all.
Unfortunately, Linden Lab doesn't give you much of support - but I guess this isn't new.
Carnin, did you get your problem solved? If yes, how? Is there any alternatives to buy Lindens from Portugal without paying outrageous taxes to third parties?
I would appreciate any help,
Clark