both sides of a lid from a paint can. i have mailed plastic lids. they have been the leftover lids from the shorter-than tupperware-lifespan containers like *glad* containers. because they are transparent, i put a sticker on, and write something clever like, *don't flip your lid.* and then put a mailing label on the other side and take it to the p.o. and get a meter strip. i have also mailed paper plates with the message written on one side and the address on the other. mail does not have to be rectangular or square, it may be an odd shape, or even an odd item. the most unusual piece of mail i ever received was a 3-ft inflated toy crayon. the address was written in sharpie and the mailman looked mighty funny carrying it down the street. i saw an article that some ladies were making a fortune mailing invitations that were written on inflated beach balls.