A company will have their adverts printed on to the back of the envelopes and you can request up to 5 free envelopes each week.
So you can send mail without paying for the postage. As for the ad, it should not really bother you or the recipient.
To register to receive FREE envelopes with US pre-paid postage
Click for US residents.
If the offer is EXPIRED, click HERE.
This offer is for residents of the USA.