Once you've gotten a bit of rapport going on during the initial approach, regardless if it's true or not, would it be worth to casually mention how you're seeing another girl (not exclusive, and not a gf) to create a tint of competition inside her. E.g. "a girl I'm seeing has that exact same top".

Or how about as an opener even. "Excuse me, can I get your opinion on this particular item. It's for a girl I've been on a few dates with. It's very casual, we're not going steady or anything but I like to be spontaneous with gifts".