Any business has to make a profit to stay in business. As long as the business is not "jacking" prices because it is difficult for you to go elsewhere, charging for certain services regardless that you purchased the item at the business is not only fair, doing so is commonly mandatory in order to stay in business.

However, if I detected or discovered that a business was penalizing me for being an infrequent customer or buying cheaper elsewhere by charging me more than is charged others, or I felt that the owner was charging me more because he or she perceived that I was not being as kind to them as they required, I would expedite finding another like business to shop at. If I did not know how to perform the services I was being overcharged for, I would be learning very fast.