Stealth. Crush resistant. Cheap. That's what I've been looking for. My preferred method has been to use corrugated plastic signboard, which is in endless supply right now -- the political signs in your neighbors' yards would probably be perfect. I Cut off a piece about the size of a small photo, put the seeds in the corrugated channels, plug both ends of the seed channel with a cotton or tissue plug, insert into a greeting card and mail it in a standard size envelop with "Photo Enclosed" on the outside of the envelope. It meets the max thickness of < 1/4 inch and seems really crush resistant. Cost: ~$1.40 within the US. I used the same method to ship to India and the cost was $2.29, so reasonably affordable. If you cut down the corrugated to about a 3 cm square, it can be mailed with a standard greeting card for 1 US stamp.