I recently bumped into a study that found "a free shipping offer that saves a customer $6.99 is more appealing to many than a discount that cuts the purchase price by $10".
http://knowledge.wharton.upenn.edu/article.cfm?articleid=1449
One way Etsy sellers can use this tactic to their advantage is to offer free shipping to their return customers. Why not create a free shipping coupon code and include it with your thank you note?
Have you tried this, or any other free shipping incentives? Have they worked for you? I'd love to hear your opinions on this!