My account balance is just over $34 but when i try to make an in-app purchase of $29.99 it tells me i have insufficient credit

I have ~$34 in my account but I cant buy gems on Kindoms of Camelot: Battle for the North for $29.99. I am able to buy gems for $19.99, then gems for $4.99, and then more gems for $4.99, but i cant buy the one big pack for $29.99. I wouldnt mind normally but the $29.99 pack has %25 more gems than if i were to buy individual $4.99 packs equal to $29.99. This has happened multiple times before but I have ignored it and bought the $19.99 instead. I just redeemed two $15 iTunes cards, so i know that i at least have $30, enough to cover the $29.99 purchase. Is there a glitch in the system or am I being cheated? what can i do to stop/fix this?