I think I figured out the trick to shipping packages at the USPS.
I don’t need to ship things often, but a year or two ago, I sold several books online and wound up paying way more than I thought I should at the post office. My mom used to send me small boxes now and then while I was at college, and the postage on them was never more than a few bucks. So why was I paying $12 to ship a book I had only sold for $20? This happened a few times, with different people at different locations, and I just wound up donating the rest of the books instead of bothering with the hassle for only a few dollars.
A few weeks ago, I was back at the post office to send DVDs of photos home to my parents. The woman in front of me was also shipping a small package, and the postal worked kept asking about shipping options for a package - insurance, tracking number, signature from the recipient, etc - until the woman said, “I’d just like to send it as first-class mail.” The employee said, “That’s the magic word!” and rang her up. So I did the same thing when it was my turn, and it was less than $2.50. Same thing happened again this morning.
So that’s your lesson for the day. If you want to ship something of lower value and just need it to get there sooner or later, ask for first-class mail. Otherwise, I think they’ll assume you want to send it priority or express or whatever, and it will cost a lot more.