The way to get money for a shipment via post office. It goes like this: you take something to the post office, and pay for this extra service, and write the amount you want. The postman delivers it to the recipient only if the recipient pays how much you said. Then the post office delivers the money back to you.
Which is why other ways of payment to postal shipments haven't taken root yet - this is too simple. Mailing a check is still uncommon and pretty much unknown.
People remarked that this is basically cash-on-delivery. It is and it isn't - this is not some special service run by some business which may or may not cover your area. You don't have to check for availability, this works everywhere, because it's your postman who delivers.
Update 2012: now there is a delivery service which does the same, which has boosted the popularity of online trade. While online payments are still somewhat problematic (your regular card won't - you need a separate one for online shopping, and you may need to send proof of payment in some other way), this is perfect: you pay only when you have received the item. Requires cash, though.