Okay, so my psn account hasn't been letting me add funds for at least a year, and I just bought a PSn card through amazon and I was wondering if the card works outside of NA
They do, but if your account is locked into a non-US region they won't.
However, you can get around that by creating a new account (which is free) and saying you're in the US region, at which point US cards will work.
Same applies for Japanese accounts and cards, (and probably others) as people figured out when trying to get the One Piece Musou DLC.
Any games or items you download to your machine, regardless of account, can be used by all accounts on the machine.