Steam doesn't add 15% to the purchase of a game with Steam Wallet, but they do add something like 15% to the purchase of an item on the trade-shop. So I'd put an item up for sale at $5.00, and you'd purchase it, but also incur the fees Steam has, probably knocking it up to 5.75 or something like that.
Not sure if Amazon is charging fees for digital codes now. I don't think they are.
We should be able to work something out......any way to find out what the cost (on Amazon) is going to be without straight purchasing it? It wants my CC without giving me a final price. If we can find that out I'd be more than happy to do it that way, would be easier for both because we should be able to match the value exact via the Steam Market and you could hold onto the money for later or spend it now.
In short I have money on my Steam Wallet, I will buy any game on Steam for you matching (or close to) the final price of Tomb Raider purchased from Amazon for $4.99 (advertised price....I've heard there's a 15% tax or something, can anyone confirm?).