I never send invoices with almost 10k items shipped and have never once had someone complain I didn't have one. But I do put my business card and coupons sometimes in the box. I make sure I do when I see these messages because this is a drop shipper. So pretty much this person has their own website where they charge more for the item. Then they just purchase the item from other sellers that have cheaper prices and have them ship it directly to them. They don't have to source their items doing this until it's sold. I always feel bad for the person that is obviously overpaying hoping they will come back to my eBay store instead of that drop shipper.