Eshop, international problems :(

I live in Costa Rica, but my 3ds' eshop is set as i lived in the usa. Because of that, i cant use my credit cards, so my only way to buy is with prepaid cards so i bought one from the usa to redeem it. But when i try to redeem it it doesnt let me because its not from my region even though both, the shop and the card, are set in the same region :( what should i do to redeem it? I have no problem in reseting the 3ds because i havent bought anything yet :) thanks for the help and sorry for my horrible english :)

thats weird, I have a Japanese 3DS and Wii U, but I have no problems purchasing stuffs from their Eshop with my US credit card