That is just point blank wrong. I have had this happen to me while doing a walk through and been charged for a higher value at the point of submission, only to have the book come back at a lower grade, and I received a refund for all amounts paid over the minimum fee. I don't know how many walk throughs you have done, but I have submitted hundreds of books now and have had very few issues with CGC, and when something comes up, I have found them to be very accomodating, including the walk through refund issue.
You could just contact the billing department and work it out directly, but I don't see the need to flame them on the boards. :shrug: