@BisonHero: Is it that they let you add money to the wallet, or that you just pay the balance of your transaction separately.
Usually I don't use Steam wallet, but I remember that last time I did, something ran over what the balance was and I think it just asks you how to pay the balance of the transaction, rather than asking you to add money to the wallet first.
I recently got an American Express gift card. For the most part, I've found these to not work at many places in Canada.
I was able to use parts of it on other purchases, and found myself with $9.20 left on it.
I often buy things on Steam, so I figured that I could put it into my Steam wallet and it would just get used when I buy something there. One problem: no custom amounts for adding money.
Using developer mode in Chrome (analogs exist in IE and Firefox) allows for you to change what that link calls, so I just changed the number to 920, and I was able to add precisely the $9.20 that I wanted to.
Just thought I'd share this trick in case anyone else ended up in a similar situation.