In app purchases have an extra tax or just a problem?

I bought a 15 dollar gift card to buy this bundle of 2600 ice (Ice like the gems in those games that can be rarely obtained or just a shorcut by buying it with money) on this game called Vainglory and it cost $13.99. When I submit my purchase it says Insufficient credit which means I don't have enough but I got $15 credit??? (I used it anyway for a small bundle in that game that cost $6.99 but this bundle was a limited one time offer so it had more ice than it used to be, then tried to buy the first and smallest bundle of ice/gems for 6.99 and still didn't work but I got $7.10 balance left). It can be a server problem or was there a ''tax''??