Gee, I've not bought a zipper at a retail store in years, I save and reuse them or buy them when I find them at thrift stores for really cheap. See if you can find out how long it took her to do the job and figure it from there? Maybe call somewhere like a dry cleaners and see how much they'd charge. I only do things like this as a labor of love. I would accept a gift from someone I'd done a repair job for, one friend I'm thinking of would offer to take me to lunch or something. If she's not desperate for the ca$h, maybe you could provide a gift certificate to a beauty salon for pampering or for dinner for her and a friend, or take her out yourself. Since she said to tell you merry Christmas, I feel like you really can't pay her for the job. It has to be a gift.
I should never overestimate my basic arithmetic skills.