I always include a hand written receipt, I use receipt books purchased at WM, I also include a hand written thank you note plus a business card,
compared to what I read this might be over kill??::):)
But I have done this since I started, probably will continue.
I received an order from etsy some time ago, the product was put into an envelope, nothing else, no business card, receipt or thank you, needless to say, I never bought from that vendor again.