Wait for her

You stay in the middle of the book store, waiting for her to come back out, watching her bare legs and the bottom of her shorts still partially in view below her shirt.

As she returns, she makes a quick beeline for her jeans, starting to pull them back on, but you're not going to let her get away with that. A quick click of your fingers and the buckle snaps off, leaving her with no way to hold her jeans up.