Natasha gives very good advice. Do what feels right for you. But just know that there are alot of us (myself included) who do NOT send receipts. I've never, in all my sales, been asked for one.
After thinking about it, I suppose it's possible that some TYPES of sales might do well to include receipts. Maybe if a sale consists of many items (especially, say, small items, like beads), a receipt can summarize all that is in the box. But I typically sell just a handful of items (ornaments) at a time. These are very individual items, personalized...so really no chance to confuse my items and with another shop/maker.