It would be highly unethical for the shop owner to list a design that's not their own on their shop without express permission from the designer. Do not encourage shop owner to do that unless it's your own design.I found a person an esty that prints mods for cases and asked them if they can print something for me if you have the file already. Best if they have some T1 stuff cause then they can add it to their store.
Asking them to print something is ok, but it definitely gets messy for them to just start making money off it.
Lots of services online: Xometry, shapeways etc