I totally understand the copyright issue, and that is a good thing for those of us who often make our profits making copies and enlargements. All you had to do was simply prove you took the images and that you are the rightful copyright/artistic owner.
Try to understand their reluctance and realistic fear of being sued and fined for printing copies and enlargements of photos that MIGHT have been taken by someone else; it HAS happened, and no legitimate business would risk a lawsuit, a fine and bad publicity.
30 years ago in Miami, Florida I encountered a more ridiculous situation at a well-known "bigbox" business. After paying a deposit and upon returning to retrieve my prints, they denied me my prints because they do not ordinarily serve "professional photographers," at least that's what they said; that constitutes blatant discrimination, I told him, since there is no such "law" and they did not post their SUPPOSED "policy." I handed the manager the business card of the attorney I worked with, and calmly said, "You'll be hearing from this gentleman after he speaks with your company headquarters. Don't forget that I PAID a deposit... and you are not reimbursing me for the money I paid nor for the film nor for my time..."
The manager asked what camera I used, and as soon as I said "Nikon," he said, "Aha! That's a 'professional' camera!" I looked at his shelves and noticed point-and-shoot Nikon cameras and asked, "Are those Nikons also 'professional' cameras?'" I walked out but he caught up to me with my packages of prints at the parking lot, apologized profusely and gave me my prints without my paying the considerable due balance.