A shoplifter is a person who takes things from a shop without paying. That is the whole of it: not the act, and not the shop. News reports and shop signs use the word (“shoplifters will be prosecuted”). It is a plain, slightly official word, not a joke. From shop + lifter: a lifter here is someone who “lifts,” meaning steals. Though actually, many shoplifters are not professional thieves — some are people who hide one small thing on the way out — but the word itself does not sort those kinds. If you need the action, use the verb shoplift; if you need the crime as a thing, use shoplifting.