I tried doing this for awhile, but I've gotten much better responses when I personalize the packaging. When I ship prints, I ship them in a fiberboard mailer (it's flat, because curled up prints ANNOY me!) and I write "Dear Mr. Mailman, please do not bend ART inside! or something similar. I emphasize the word "art" by writing it in bubble letters lol :)
Then, I write a personalized note (as well as sending them a message after they order) and I thank them and leave little doodles or something funny or cute that's relevant to the item they purchased. I keep it light and I write things that I hope will make them smile.
If I've done my job correctly, from the time they've ordered to the time they've opened their package, there should be 5 or 6 instances where I've made them chuckle, smile, or remind them how grateful I am for their business.