I almost feel silly blogging about these little thank you notes since they so simple, but thought I would share what works for me and my Etsy shop.
First, start with 4x6 scrapbooking paper. (You can use larger sheets, but this size makes it so convenient since there is no measuring- just one fold, one cut and you're done.) I purchase my paper on Etsy as I love supporting other shops. You may want to pick a specific color/theme to match your shop's ambiance, or if you sell a wide range of products, get a variety to suit the individual customer and their order (e.g. I'll send a funky thank you note with funky Owie Bags, a pink & chic note with pink & chic wipes, etc.)
These cards are just large enough so they don't get lost in the package, but small enough where you aren't wasting paper. I can either write a simple "Thank you! ~ April", or squeeze in a personalized note or memo if wanted/needed. Eventually, I would like to get a custom stamp so I can put my logo on the inside top. Another idea would be to get a "Thanks" stamp (which are widely available) to do on the outside, assuming you are using a lighter color paper.
I bought my last batch from BethMarie10, and the resulting thank you notes only cost 4.5 cents each. That's a small price to pay to be able to give my customers the warm & fuzzies about both my shop and buying handmade in general. <3