Big difference pbadder, the legal size is a #10 envelope, measures 4 1/8" X 9 1/2". Walmarts, grocery store or if you have a dollar store they have them too. 80 strip & seal for a $1 or so. Think I'd be changing envelops for the return trip.
Arizonacat last swap I know a solid black was sent, but she did add a note saying "use bright solids". If you didn't add something you could have all kinds of prints and such that they not match up at all in the end. But it would depend on what you're going to do with them. Whatever you decide will be great, but a little hint would be nice.