I'm in another field then advertising, but less then 5% of transactions require special handling, but they represent a significant investment in error handling, procedures, accounting and reversal mechanisms. Even then we are still manually dealing with things that represent 0.00001% of activity. Accounting correctly is always a deep rabbit hole, it comes down to how much you plan to right off inside your margins. :) Thanks for writing up this inside perspective of running this type of business.