I don't think you can be guilty of shoplifting if you pay for the merchandise.
You most definately can. As an example, some years ago I owned several convenience stores. One night a drunk walked in, grabbed a $5.00 bottle of booze, threw down a $10.00 bill and walked out. He was charged and convicted of shoplifting. There was no "meeting of the minds" required for a purchase.
A judge would probably look at the circumstance of someone refusing an exit check but the store has every right to press charges. The terms of the sale have not been consumated.