I've noticed a possible bug in the Custom Fields section.
If I create a fee-based custom field first, and then decide later to edit the custom field to take out the fee, the change does not stick.
I've tried several ways: delete the fee first and then uncheck the box, click save. Or uncheck the box first (causing the fee to be grayed out), click save.
In each case, after I've saved, I noticed that the fee is reinstated in the custom field. The change does not seem to stick. I basically have to delete the custom field and start all over again without a fee.