You mentioned you're using a Visa card, so most likely the problem is with your bank/credit card company, assuming you're American. Payments to Mojang are handled by/routed through a UK-based business, so most likely your bank's fraud detection is set up to block any overseas purchases without prior approval. You just need to call the number on your card and let them know the purchase is legit.
hmmm...I dunno,i'll contact them,but i'm 100% sure it's not the VISA at all,since two of my friends bought it using VISA card,i honestly dunno.Oh now i know that the problem is from Mojang,everytime i put on my visa it just says unable to make transaction,i tried putting in a jumble of numbers and it still said the same thing...After that moment i knew that it was Mojang not wanting to handle any of my transactions.(Plus i tried using another pc,nothing,another browser,nothing.)