I bought these on fictionwise because they are 100% credit, but it didn't seem to work out that way?

I bought a buywise account, and 2 of the books, giving me 18.05 in micropay.
I then used that money+my coupon to buy the ommnibus. I now have 6$ in my micropay, I was under the impression I should have 16, since that would be a 100% rebate?