The sender's address is the return address and it should be placed on the front of the envelope in the upper left-hand corner. The delivery address as well as the postage should also be placed on the front of the envelope.
The United States Postal Service website suggests that addresses on envelopes be printed in ink and that no punctuation be used. This will help machinery to process mail along in a quick and efficient manner.
Using the correct ZIP code helps mail to reach its destination faster. Mail that is not addressed properly may be returned. Postage for shipping mail can be purchased at any USPS location.