According to the U.S. Postal Service, padded envelopes, unpadded envelopes larger than 11 1/2' x 6 1/2', metered mail, and mail marked for hand canceling and sorting *should* avoid the ravages of automated postal machinery. It is, however, prudent to protect seeds with some sort of soft padding: folded paper towel, bubble wrap, etc. If you are sending out several packets of seeds, it is helpful to tape the packets down individually to an envelope-sized piece of cardboard before wrapping with padding so the packets don't shift on top of one another, making a bulkier envelope that is more easily damaged. Regular letter or legal size envelopes should be kept as flat as possible with minimal padding to avoid being torn in transit.
Generally speaking, large, soft, and/or very fragile seeds require more careful packing and will probably travel safest in a padded mailer. You can save and reuse any padded mailers you receive to help lower expenses. International trades should be sent in padded mailers. The United States Postal Service requires a customs declaration form be filled out by the sender, identifying the contents and declaring a value: the simplest is to put 'flower seeds' and 'gift - no value' so no duty fees will be assessed. Seeds crossing international borders must be individually labeled with full Latin names. See the FAQ regarding rules and regulations of shipping plant materials for more information.