Sometimes the greatest art is in getting the Post Office to agree to carry an item, but in our town the rules are simple; if it can't hurt anyone they'll take it.
Some of my mail art threads have been running for decades. For me, the main objective is that the item be visible to everyone who handles it along the way.
I've sent beautiful envelopes, ordinary envelopes with strange stamps, and unwrapped items of every description-- paintings, drawings, sculpture, books, clothing, toys, pots and pans, weird miscellany, tons of rubbish like candy wrappers, paper coffee cups, or a bag of pencil nubs.
I sent a rotting bouquet in a plastic wrapper. "Nearly Fresh Flowers" the label proclaimed. I got back a knotted cloth napkin with a sad face drawn on it. In the world of Mail Art, you always get better than you gave.