If this is the case maybe you could send in/take in a basket of muffins or donuts with a thank you card. That way they aren't personally accepting a gift but it's still something nice that you can do (and it's just a morning/afternoon tea for the staff).
I like this idea that way they could share with the staff that they work wit or maybe call the Apple Store and speak to the manager as you wouldn't want to be embarrassed if you rocked up and they wouldn't let you leave them
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