Unless she's the owner/manager, I'd absolutely write a formal letter of gratitude and appreciation to her superior. It will go in her personnel file. Indicate that you are so impressed with the service "above and beyond" and the customer care that you will continue to bring your business to this store.
If she's working retail, she's not making a huge hourly wage, I'm guessing. I'd give her a gift card to a broad spectrum store of some kind (but not one that competes with her store's products). I'm not sure I would do anything like Starbucks - it's so expensive that a lot of people making minimum wage just can't afford it, so buying her the equivalent of one cup of coffee doesn't express your gratitude. If you do a restaurant gift card, make sure it's in an amount that allows her to take herself and a guest, plus tip. Otherwise it's additional money out of her pocket just to use your gift!
Gas and supermarket cards aren't glamourous, but everyone can use them. You could also call her manager directly and say you'd like to do something, and what does he/she suggest since you don't know her personally. Otherwise, I would do something that's not too personal, like a bookstore gift card or a green plant she can display (perhaps) in the store or the employee break room (complete with a big "thank you for your outstanding customer service" card stuck in one of those florist picks) which she can also take home with her. If it's in a nice pot or basket she can use for other things, that's nice too.
