borntobesting: maybe your credit card company is adding a small fee for smaller purchases.
Most likely ^^^
Read your terms on your card. If it is a prepaid card, then it may very well charge you a fee for using it. Processing fee.
If it is a standard card, then there may very well be a minimum charge threshold. You did not mention how many credits you purchased.
One last possibility. If you are from outside the U.S. you may be charged an international usage fee. I had my card company drop that, as I travel to China. 3% international fee, adds up, quickly.