There is a 3/4" maximum thickness that you have to be under, or the envelope becomes a parcel, and rates go up. Altoids tins don't make it under the 3/4" limit. I'm not sure if the requirement was there before, but they never checked this when I have returned swap packages in the past. The last one I shipped out had approximately half of the packages needing between 40 and 80 cents additional postage each when they did the 3/4" check. Not a big deal financially, but it really ends up taking a lot of time at the PO counter. Be kind to your swapmeisters, and double check your return packages before they go out.