You can always go and ask for "cheapest w/ tracking" using the tall, thin brown paper envelope (not padded) they sell for $1.19, also. If it's light enough, it might only cost around $4 including the envelope.
Or send in their small flat rate box for $5.45 or whatever it is. Just fold in half and roll if you have to. Including tracking is always the safest bet which protects you from buyers trying to keep the item AND the credits, but try to do it as cheap as you can. Always have the clerk weigh it before you decide on a package, unless you already know it's heavy (such as shoes or sweatshirts), and then it automatically pretty much goes into a flat rate packaging. If it's fragile, pad with a bit of bubble wrap, then always send it through the clerks, that way it can avoid going through the various sorting machines which can crush the item and/or rip open the packaging. Have them add a "fragile" stamp and/or "non-machinable" if it's a smaller envelope, just in case, to protect the item and yourself.