I'd suggest you do it ingame,
![](http://puu.sh/mxF9r.jpg)
From single player click the gold coin next to your balance on the bottom left corner and this UI should pop up ingame, click your desired amount and a steam message will pop up(In my case it is asking me to add the funds, but if you already have the funds on your steam account it will properly just ask for a confirmation)
This method will only work if you are running the steam version and NOT the standalone(?).
I won't get started with how you should probably not buy from nabi, and go with buying it from an other user with paypal because you mentioned that you have already bought the steam gift card. :P
Also I've been looking into your issue on the website, I can't recreate it though, you did authorise the payment on the steam website right?
The steps it goes through are:
You log into steam via toribash > You get taken back to toribash to chose the amount you want > You get taken back to steam(you need to click the link after you click purchase) you then have to authorise the payment:
Last edited by Tuna; Jan 16, 2016 at 11:24 AM.