I haven't had any repeat customers thus far, but that is awesome that you have found such a loyal fan! :) If she refuses to use a coupon code, a little "thank you" item in her next order may not be a bad idea.
Whenever I send out an order, I always enclose a hand-written "thank you" note along with a coupon for their next purchase. Of course, no one has used it...but it's there :)