Thanks so much for the advice and suggestions.
thedots, I really like that idea about adding the feedback request to the already generated email that the buyer gets when they purchase something. I will definitely add something like that to my message and that way, I don't feel like I'm bothering them.
When I mail out my item, I include a little note thanking them for their purchase and if they are not completely satisfied, to please contact me. Then I add a little note about how much I appreciate their feedback and give them an incentive for 15% off their next purchase. I just started doing this so I'm not sure if it works yet. I got this idea from another shop on Etsy but after reading these responses, maybe it's not appropriate.