I bought card from 4saleusa and contacted PSN Support they said they have temporary system problem and to try in next 24Hours.
I have the same issues.
I've bought three prepaid cards, $50 x2 and a $10.
First I got the 'An error occurred (E-82000002)' message. Then I got the E-82000102 message. Tried a couple of more times and it went back to E-82000002. Now I'm getting E-82000156 saying the codes are no longer valid.
That's when I bought the $10 just to check, and I got the same message (not valid). I'm guessing adding funds to your wallet has been temporarily disabled, but it would be nice to know when the feature was bak online..