i mail all of my swap squares in tyvek priority envelopes from the post office, they hold up to any abuse the postal workers put them through and are (weather proof). usually costs $5-$6 to send them.
they are kind of boring blocks to make, and it is remarkable that i'm doing it since i pretty much NEVER make the same block twice...but i found they go together really fast, when i first signed up (on a different forum) for this same swap i went in and had 100 stitched in about 1/2 an hour, clipped the corners (when i stitched them i sewed on the diagnal line and then turned around and stitched 1/4" outside the line so when i clipped the corners i had 200 1/2 square triangles done at the same time) tossed all the little blocks into a large ziplock grabbed my pigma pens and headed to work...when i first started signing them after about 15-20 i thought, I will never get these done...writing on them is the worst...but even that got easier as i went along and i had them all done in that one shift. the motivation was the vision of the quilt they will become :) but i have found for just mindless filler activity they are great! i work in the private duty medical field and have 'down time' ...time when the client is asleep and i just need to be awake...sit quietly and sew... if i try to read my book i fall asleep, so sewing all night, or working on projects like this makes my time go by so much better. i work 16 hour shifts, and that can get LONG but with these blocks, if my client gets up it is so easy to just set the block down and go back to it later, no worry about where you left off. between the 2 forum swaps i'm doing these for i will have about 225 siggy blocks, and i have a whole bunch of scrappy little 9 patches from a swap a few years ago i never knew what to do with...evidently they had a purpose, i just wasn't there yet...they are going to go so well putting these together.! now i am excited about these!!!