I wouldn't rely on any service that is not trackable nowadays as I have seen some very shady fellows in USPS/UPS/FedEx.
Once my employer lost a $20k check and have to call the customer to stop it, then our mailbox was broken into, then I got my neighbor's playboy for a few issues (yeah, bring it on), and they got my PC magazines (fair trade huh?).
Online payment is the only way to go now, the onlything USPS is good for is shipping out heavy and cheap stuff (like textbooks) on media mail that you don't care about paying high cost for shipping and can afford to lose it.