A certificate of mailing, as provided by the
United States Postal Service, is a small
piece of paper with a
hand cancellation mark on it that states that you have
mailed an article of mail with a
postal clerk. It is considered by the USPS to be an
official record, however that certificate is the
only record you mailed something.
It is useful if you simply need to prove that something was mailed. If you need evidence of receipt, you use certified mail and tack on a return receipt for good measure.
Cost for this is 90 cents.