About ZP!

Dear CrossFire, as you know the purchase of 20,000 zp is $17.49. A few days ago i bought 3 Ultimate Game Cards that were $20. When i redeemed them and purchased 20,000 zp it said i have $2.51 left on my account. I had three Ultimate Game Cards and purchased three times 20,000 zp. So 3 times it said i had a total of $2.51 left on my account. That means i should have a total of $7.53 on my account. But i dont know where to see this left over money on my account or what to do with it. Please help!