One of the things I pride myself on is using padded mailers that are made from recycled materials. I also take delight in decorating each one with my "Jackbear" stamps, which are bear tracks. I package each stamp I sell carefully in a small ziplock bag, with a business card stapled to the top. I also include an index card with the stamp's image on one side, and a personal note to the buyer on the other side.
I have not been including invoices because I believe if my buyers on Etsy need a hard copy, they can print that off of the Etsy site. I don't want to waste another piece of paper if the person doesn't need it. This fits with my efforts to reduce the amount of waste in the mailing process. I hope that my buyers understand and appreciate these efforts, as we all try to do our part.