Hooping.tribe.net (which has nicely filled the hole left behind by the hooping.org forums - RIP) has a great discussion about mailing hoops via USPS. Eric had the definitive answer about the issue:

You can ship them USPS cheaply if you wrap them as a ring and not a solid circle. In other words, rather than boxing it, wrap it with paper as you would hoop tape. Then the measurement for the 'length plus girth' is equal to the diameter of the hoop plus the circumference of the tubing. That keeps it from being an oversized package and you just have to pay for the weight, which is minimal. The only remaining challenge is educating your friendly USPS worker on their own guidelines!

Jasmine then piped up with the confirmation:

yes, the education must happen. my post office finally got it, and though they groan when i walk in the door, they smile too. the trickiest part is weighing the hoop on their scales. but shipping it via priority mail should be between $5-$9. super cheap. just act like you know what you are doing and hoopfully they will follow along. good luck!