Small bubble wrap, I cut it the size of an envelope and put seeds in between, never had an issue sending this way.

I dont usually bother padding the seeds that I send. Never had a complaint but I am sure some seeds were crushed in the auto sorting process. But, the bubble wrap is a great way to ensure they don't get damaged. I have seen some send them between a couple layers of cereal box cardboard. Some people use small bubble wrap lined envelopes or other padded envelopes as well. I usually send enough so that if a few get ground up, it isn't a big deal and I send them inside their individual coin envelope with a folded sheet of copy paper surrounding them before putting them in the mailing envelope. If I was selling seeds, I would go the extra mile, but I am usually sending free seeds or trading at the most.
Bruce

.soapbox mode on

Please be sure that you label the seeds well. Too many times one gets an envelope with the reputed name of the variety and nothing else. Please include the year in which they were grown, whether open pollinated or isolated, who grew them, and any other notes that may apply. (I also list species where known.)

One of the coolest packets I got was one with where the sender had used a color printer to print sticky labels with the picture of the (peter) pepper and relevant info. Still have that envelope.

Some people manicure their seeds. I don't mind the junk at all, so long as it's dry. (That's the only real problem with junk - it holds moisture).

.soapbox mode off

Dennis