Singles could go in top loaders (yes, they make top loaders that big) and if you have a lot, maybe inside a cereal box in an outer shipping box. Nothing is going to be cheap in this case, other than between two thick cardboard pieces in a mailer - but you need to make sure that cardboard can't bend.
-- Dan --
Note: Please see my profile for more info regarding trading (section updated 3/4/2024). I have added a large portion of my inventory to the site, and currently have trading turned on (details are in my profile).