How nice of you! People in these jobs are so under appreciated! (We tip our letter carrier, the FedEx/UPS drivers who trudge through the snow to deliver packages with a dog biscuit for our pup, and our garbage pickup people.)
I know you're in Florida, but she's outside in February when I know Florida people wear scarves and gloves. How about those combination gloves/mittens with a flap that folds up over fingerless gloves, allowing her to take tickets and make change but then cover up her fingers in between cars or when she's out at the gate. Or a pretty and moderately warming infinity scarf so she can be a little fashionable without having her coat zipped up to her neck all the time?
A travel mug is fine, or one of those electric heating "coasters" that keeps a regular mug warm.
Maybe she would like a little basket of lip balm, hand lotion, hand sanitizer (I hate the stuff most of the time, but then again, she's touching the card reader all the time), candies or mints, pocket tissues, and air freshener (if she's breathing exhaust all day). You could get some things at the dollar store or thrift store and make a cute basket she can display and replenish - if she's allowed to have any "decor" in her booth. It wouldn't be much money, and it would show you were willing to take the time to create something personal.