You can put a plate block of 6 or 4 in a brown kraft envelope with a cardstock sandwich and mail it letter rate if it is less than 6-1/8 inches high x 11-1/2 inches long x 1/4 inch thick. It can't be too rigid. That will save a lot on postage. offering a low cost shipping option is good practice for cheap items. I just bought a plate block last night for a thematic page I'm making that was 1.45 and had free shipping on
ebay. (the 1971 Historic preservation block of 4) You're not going to get rich selling this sort of thing but some folks will try and add additional things to make it more worthwhile for you.