As much as this is an amusing exploit, the shopkeeper probably should have some reaction to customers messing with the merchandise like this! Maybe a dialogue from them, something like "Hey! This isn't a library - stop messing with the merchandise!" The shopkeeper then moves onto the entry space of their building and insists that the player pay something toward the unpaid pieces or else drop the completed map before leaving. If the player refuses thugs are called as normal. Even more painful - mark the full map as "unpaid"..

Marking the full map unpaid would be quite evil. I think the shopkeeper should just prevent the merging of the map.

I like Keli's evil suggestion to make the entire map unpaid. Although as a shopkeeper I would surely warn the player before.