When I have mailed things for swaps in the past, I wait to seal the box until I find out what the postage is and then include cash or check for the same amount or round up to the nearest dollar. I am fine with either.
If you don't have a padded envelope, you could use thin cardboard (like a cereal box) to keep the blocks flat and put in one of those "plastic" envelopes...can't think what they are called. That is how Mirabelle sent hers and they arrived safely from AUS.
Jan--don't over-do it today out side! Make sure to get a little help from MrJanRN.
Off to archery this morning and then hopefully sewing this afternoon. It is in the mid-30s today, but supposed to cool off again tomorrow.