Mailed some small MC parts to Puerto Rico a few years ago, they disappeared. Buyer dinged my paypal and got a refund. About 6 months later the parts showed at his door they were lost in a crack somewhere he re-paid and we were both happy!
If you do much shipping at all, get a post office online account and print your own shipping labels at home, the rates are about 10 to 20% less. Post Office takes Paypal too. It's also the best way to deal with an insurance claim.
Your mailman will come and pick up (almost) any size box you want to send. The post office is happy to send a variety of Priority mail shipping boxes to your door no charge. Rate charts have recently changed, shipping larger items in your own box has gotten less expensive at the Post Office. When checking a price; select the "your" box option, give weight and size even if you will end up putting it in a priority mail box, there are some less expensive shipping options that you won't see if you just choose a flat rate box. For a couple of reasons I try to ship less during the busy holiday rush.
One post office "secret" is regional priority boxes, they only show up as an option if you select the "your own box" option. Your local post office doesn't stock the regional boxes but the post office will send you a selection of them, free of charge.
As always pack your items well, no shipping service handles your package with TLC! It's going to get slung, shoved, and dropped.