Panties are lighter than most letters.
Step 1: buy some white letter envelopes that take half a sheet of printer paper. (6 x 8 3/4 inches). The ones I have use a pull-off thing to get the stickey bit. Buy some forever stamps, and ziploc baggies.
Step 2: prepare the panties however you do it (shoving the crotch up my hooch to get my scent on them is my method, some girls will just send them clean, and some will spray with perfume. Find out before hand if your guy has any perfume allergies if you do the latter... I promise they happen.) LET THEM DRY OVERNIGHT. This is important because wet + dark place = mold. If the guy you're sending them to is allergic to mold, this is a HUGE problem.
Step 3: fold the panties so they'll fit in the envelope (you may need to test this a few times before you get it right, but the envelopes are cheap, so don't worry about wasting one.) Put them in the ziploc, get all the air out as you close it (some girls suck the air out with a straw, I flatten them under a pile of textbooks). You want them really flat to use this method.
Step 4: address and stamp the envelope BEFORE you put the panties in. One forever stamp is enough, but if you're really nervy about it use two. You may want something heavy on hand to help hold the envelope flat while you close it. I once had to tape the envelop shut, and had no issues at the post office.
Step 5: drive to another town, and deposit in a mail slot.
Please note: I used to have a PO Box, and the first time I did this I did verify with the post office that the package was small enough for a regular stamp (it had my return address, but the guy did get the panties just fine). Since I wear cotton bikini cut, the only heavier are briefs, boyshorts, and VERY frilly thongs. I'm pretty damn sure briefs and boyshorts will be light enough.
If anyone has a kitchen scale, the regular stamp can handle any "letter" under 3 ounces to any destination within the US.
EDIT for clarification