Most of my clients prefer to receive their invoices by email now, but I still have one or two that prefer the printed format.
I've been using www.l-mail.com
now to send most my printed letters. They are cheaper than air mail for international post, and for domestic mail I figure its cheaper than taking the time to go to the post office.
The interface is good as you can format your documents, but if you have HTML invoices or letters, you can cut and paste this into the letter so your letterhead image can appear (in black and white) okay.