I always wrap mine is a tissue paper and wrap it with a ribbon. I am working on getting stickers made because I worked at an upscale boutique and they always sent packages like that. I hand write a thank you card, include a business card, and add in a little something extra that I've made- I've sent felt Equestrian type ribbons I've made, hair accessories, necklaces, etc.
I have a question for you guys too. How many people put the actual etsy invoice into their packages? Or do the majority of people just send a hand written thank you, or both?