When I brought my truck in for service, the document Tom had me sign said the cost would cost around $85. The final cost came out to $123. Some DEF was added, but that cost was only $10. Your cashier told me that the $84 number doesn't really mean anything. I realize the the final cost will sometimes be higher. But it is all about managing expectations. If I am told, in writing, that the cost will be X, then unless a tech comes to me and says additional work (and cost) is requested, the final fee should be close to the quoted original fee. Otherwise, what benefit to the customer is the initial paperwork?