I would simply tell them that the high minimum is for Customers not Clients. Since they have told their Reps that we are to be their go-to Cleaners I would write back and tell them forget the invoice...that this one is on me and then carefully explain why your minimum is justified in the future or...lower it to the 85 dollars they were expecting. Was there an opportunity to up-sell on this job? Something else in the home that could have been done as a separate job? Regardless... for a Client that will refer you good jobs in the future the 175 dollars seems like a small price to pay. In my opinion :-)