Can you make it so it sends us an email reminding us to snail mail it, say a week before it is due? Currently, it is $1.20 to snail mail 50 of them, which is very expensive.
Also, you should be able to find a company to automate and setup two printers, one for the invoice(and staple) and the other for the envelope with the address. Possibly a robot to scan both items to match them and put the invoice in the envelope to be ready for mailing.