I don't do invoices.
I thought about it when I first got things started on etsy, but my printer was busted at the time.
To be blunt, I've always considered invoices to be a waste of paper and ink, and I really hate getting them in packages I order because there's soooo much printing just for the one little thing I ordered. I could see if I was ordering fifty things at a time from fifty different shops and might forget what all I ordered. But, I don't.
From a seller's perspective, I cringe every time my printer needs a refill. The paper and ink costs add up quickly, and all you're doing is duplicating what already exists on the etsy pages of both the buyer and seller. What's the point of that? And because I use Paypal to collect the purchase price, I have most of the transaction information duplicated there just in case the etsy site crashes and all that info is lost.
I just don't see the need for a hardcopy invoice.
I do, however, include a nice hand-written thank you note for the customer. It started out unintentionally and has become something of a trademark for my shop. Not so much the notes, but the cardstock I use and how I attach them to the ironware.
Anyhow, no on the invoice and yes on the personal thanks. I think that's the right combination for me.