In the end the shop owner is the one who knows his books. In this example the owner had no problem giving a discount to a marine, I'm sure he didn't loose any money on the deal. Just how much he made after that shops costs were covered is known only to that owner... and the IRS.
In other words a business owner will not go below what [s]he feels is needed to stay in business and a fair profit. They are running the business let them keep the place in the black, all you have to do is give them a fair shot at your patronage.