Bet you have never heard of these. USPS doesn't push them as they would rather sell you certified mail at about five times the cost. Certificate of Mailing ( PS Form 3817) says you mailed an item to a certain address on a certain date. It's legal proof that your item was mailed. Nice thing about them, is recipient does know that you have proof of mailing. With Certified Mailing, they can refuse delivery. When I worked for mini storage company, we used then to notify customers of impending sale of their unit for non payment of rent. If they were sent certified, and they refused delivery, it meant we had to pay sheriff to go issue a summons, adding to our costs,and allowing for at least another month's rent to go unpaid. Saved my butt once when I paid child support. Started out paying by check, which each month I got back in statement. Then they converted to money orders only for payment. Bought money order, and put it in mail with certificate of mailing on correct day. You don't get any notice telling money order has been canceled. Sheriff serves me summons for non support. Day before I get to court, money posts to my account. When it's my turn, show judge my copy of money order and certificate of mailing. Tell judge ask how many in court room had their money post the day before. Most hands went up. Told judge he had a problem in clerks office with an employee. Judge apologizes to me and tells me to go home. He then recesses court, gets Clerk of Court, and they proceed to fire a certain employee. Had I not had my certificate of mailing and receipt, the results most likely would have been different for me and a bunch of others.