Good one, Gustav...
I run a mail order business and I can tell you that the US postal service is POTENTIALLY very nearly 100% reliable. The reason I say "potentially" is because they are not clairvoyant, and so if you mis-address something, address it illegibly, pack your parcel carelessly so that it opens or falls apart in shipment, or address it to someone's no-longer-current address, there is a possibility there will be problems.
Actually there have been instances in which I've mailed something that was incorrectly addressed, and the USPS figured it out and re-routed the package to the correct destination.
I've mailed thousands of packages, and the only ones that have ever been lost are the ones that -- due to my error, or the error of my customer in typing-in their own address in the order process -- were not addressed accurately.
I'm sure there are problems that are not due to the fault of the sender, but they are nowhere near as common as people like to make out. Ask anybody who sends a lot of mail, and they'll tell you that if you make out the mailing address legibly and with a valid address, the package will find its destination 99.999% of the time.
We Americans like to criticize our mail service just like we like to criticize our police and politicians, but I think the fact that you can give somebody an envelope and 34 cents and have them deliver it very reliably across the country, is a pretty great system.