This post is for mailing "rules".
BUBBLE MAILERS - Most participants are requested to use bubble mailers. A size "0" ( 6" X 9") is best for tomatoes/peppers and small flower, fruit, veggie seed, with maybe a few small packs of beans. Size "000" may be too small, since you may be receiving back more than you sent in. Bubble mailers will be recycled, with your bubble mailer likely going back to you. Of course, the "0" is not large enough for some donations.
With some international participants, a further discussion will be needed to see how many packs of seeds they would like. Using "greeting cards" instead of bubble mailers saves a lot on postage costs.
LABELING - If you're taping on a label (or using a peel and stick) make sure it is secure. Good quality (heavy duty) clear packing tape is the only thing I use. Low quality clear packing tape is OK for labels, but tears easily. It's not great for sealing packages, unless several layers are used. "Transparent" (1/2" or 3/4 ") tape can be a disaster.
POSTAGE - Domestic postage is $2.54 for up to three ounces, $2.74 for up to 4 ounces, $2.94 for up to 5 ounces. Many, if not most, MMMM packages will likely be under 3 ounces (I think). Putting the return postage ( cash/stamps only) in your bubble mailer before anything else goes in, makes it easier to not forget it.
Those participating in the Bean/Pea/Other Legume swap by itself, or with the MMMM, will likely be using Flat Rate padded envelopes ($6.80 postage last time I looked).
Next will be your paper send-in and wish list.