I've had this conversation with my SA, and he told me that they are allowed to accept food delivered to the boutique, which is always shared with the entire staff. So, I would stick to that approach to express your appreciation.
I am wondering.....Would you give something to the lovely lady who helps you find your size or fit by regularly bringing you clothes while you are in the dressing room and that....several times a year at .....Zara?
I had an incident last year where I wasn't feeling well and actually FAINTED in the store. All of a sudden I was dizzy and the next thing I knew I was on the ground. Everyone was so kind to me, got me juice and candies and talked to my husband on the phone and my SA walked me to my car. I was mortified, but they were so kind, even the store director. The next day I dropped off cupcakes for everyone and a thank-you note, and they couldn't have been sweeter.