If I were you, I'd go speak to this person privately and just explain that you find that this is less of a favor and more like backdoor extortion - paying for services rendered which you never approved previously but acknowledged when they were received.
All depends on how important and profitable that future business relationship really is. If in the long run, the unexpected costs turn out to be nominal in comparison to the big picture, than it should be handled a bit differently and more professionally. If not, then they should be blasted!
