Fit the jeans into a USPS Tyvek envelope. Do this by folding the jeans into quarters or fifths. The trick is to make the item small enough so that when you put them in the Tyvek envelope, the envelope can double over on itself. Tyvek envelopes can withstand the harsh shipping environment more than ordinary cardboard envelopes. They are also slightly weatherproof.
Now, place addresses and labels on a cardboard flat rate envelope.
Insert your Tyvek package into the cardboard flat rate envelope.
Make sure the pants are at the very bottom of the cardboard envelope and then attempt to close the envelope. It is easiest to close the envelope on a table, not in your lap.
After successfully closing the envelope, put USPS priority mail stickers over the corners to make sure they do not catch on anything.
Put a piece of packaging tape over the seal, to make sure your package does not rip open. Maybe also put another piece of packaging tape over the addresses so that they are protected from the elements and do not wear off when exposed to moisture.
You may now mail the package.