Yep, a gasoline or JoAnns gift card are great ideas. (As a seamstress, I'd personally go for a JoAnns .
I love fixing up things for people, but feel awkward if they try to pay me.
And yes, a zipper like that is five dollars or less, and 36 in. hmmm...I'm sure she's awesome, so, based on my experience, it probably took her about fifteen min, more or less. I've put 20-22in zippers in dresses, in about 10 min, and as I'm not a pro, so she could have been a lot quicker.
It might be a nice idea to wait for a holiday, like Valentine's Day or something, and give something to her then. That way it doesn't seem like you are trying to pay her, but giving her a nice gift and a "thanks for all you do".