Tell them no. They have no legal recourse to search you, and since you bought <whatever>, it's your property, despite the fact that you haven't left their store yet. I usually just don't respond to them at all, or, at best, tell them no.

Or do what I do. Refuse to shop in places that do these things. Works quite well.
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Heather

"I distrust those people who know so well what God wants them to do because I notice it always coincides with their own desires." -Susan B Anthony