Thanks for the link. I actually ran across this article while hunting down a solution yesterday, and it definitely seems like the best approach, given the constraints. Our situation is unique in that the client allows orders to be placed (online) that aren't paid for until the customer shows up at the retail establishment. This opens them up to a lot of orders that are placed, but then cancelled, and no financial transaction truly occurs. Our hope was to find something akin to an order cancel that could happen with a tracking call that would undo the transaction, and not require any back-end reconcilliation. I think Google Analytics supports this (reverse an ecommerce transaction), but we couldn't find anything related to passing in negative quantities or transaction amounts in the Adobe forums, so I don't think it's supported.
Thanks for your help!