I've never had this problem before, and I've used this same payment method many times in the past, just as a FYI before you read on.
I went out and grabbed a $50 Prepaid Visa as I normally would if I wanted to buy some stuff on STEAM. It's activated, it works everywhere else, but when I try to make a $20 purchase (I still have about $45 left on the thing) on STEAM (via the wallet, since I'm grabbing dota 2 items) I get a (paraphrased) "Sorry, something went wrong. Please contact steam support.", and a link to this area.
Any advice on how to proceed?
EDIT: Also, could/does this have anything to do with server downtime or something?