Wesing,There are as many ways to package as there are swappers. But the primary idea is to keep the packages "thin" so they cost less to mail. I layer mine in two columns of 25 then either put in a card or on a sheet of tag weight paper with my message of birthday greeting. It is common to do something to contain the squares just in case the envelope is damaged. Some wrap their selections in saran type wrap and I have received others a clear sleeve of some kind. Don't know where these are sold, but would to. Some use Dollar Store greeting cards, others standard business size envelopes or other envelopes.
I use the 6x9 mailing envelope, Dollar Store, I had to remove the metal clasp. My USPS folks told me this is a must and to then tape closed the envelope flap with scotch tape. This allows the envelope to go smoothly through the automated sorting machines. Mine send for less than a dollar each.
The post office has a little slotted measuring tool they use to measure the thickness of envelops. If you package does not slide through the slot easily, you are charged as if the envelope was a package. (Rationale for keeping your package thin.) A package mails for about $3 plus dollars. Folks in this group have said in the past that they mail for about 80 cents each.
Keep watching here. I am sure there will be loads of other good packaging and mailing ideas posted as we get closer to our finished list and start sending out the first packages of birthday treats. I am always looking for good ideas too.