Pay for parts up front?

Having been in the RV maintenance and repair business I can assure you that the shop has a very legitimate reason for wanting some form of financial assurance up front. You would be stunned if you could see how many people go in someplace and order and then for whatever reason decide to do their ordering business and fix somewhere else. While you are not their banker they as well did not take you to raise. Paying the full price is somewhat unusual but a non refundable deposit of some percentage, say 25 to 50% is not out of the question.
My response to a refusal to provide a deposit for the ordering of something that I would not normally have in stock would be a request for you to take your business elsewhere.