The "free" envelopes from the PO will cost you a minimum of $4.75. More than you need to pay for a yard of fabric. Unless you are shipping more than 13 ounces, you might want to get your own envelopes and pay First Class Rate.(max $3.41.)
i would not recommend using paper envelopes however, they tear too easily, unless you are sending something very light weight. I get poly mailers (plastic) online for about 7 cents each in lots of 100 or more. Not sure where you can buy them retail.
PS - do not count of the PO clerks to tell you the best way to send something. They are trained to "upsell" and make more money for the PO.