I usually also send mine in a ziplock inside a padded envelope. Or I'll put the damp paper towel in a small test tube, then in the padded envelope.
During non-frosty seasons I have received gemmae in probably the most-economical fashion. Damp paper towel in small ziplock nested in a 'cardboard cutout' in a regular (unpadded) letter envelope for the price of a stamp. When I say 'cardboard cutout' I mean that a piece of cardboard that will fit in the envelope has a section removed the size of the ziplock. So the ziplock (laying flat) is nested in the cardboard. The idea is the cardboard is thick enough that it has a buffer substantial enough to still protect the gemmae from the USPS roller machines that do the mail sorting. I will say that there is also some risk in this method as one (of three) shipments I received had damaged gemmae. You could make a double-thickness of cardboard (still with the cutout). I think anything < 1/4" thickness can go in a regular letter-sized envelope. Charge a little extra for shipping, and be willing to re-send gemmae in the event of an accidental smoosh?
It's an option, but not with complete guarantees. I also think you want to be sure and have a small ziplock (~1x2" or similar) so that the force of the rollers mostly is born by the cardboard, not the ziplock with precious gemmae.
[I can try and take a pic if my narrative isn't clear. Let me know]