Collect On Delivery.
This is another ancillary service provided by the US Postal Service, UPS, Federal Express, or anyone else who might carry parcels, letters, or other miscellanea as such, which instructs the courier to retrieve the payment for the article at the time of delivery or within a certain period after the article's arrival at their central delivery location (post office for the USPS, etc.).
USPS will collect the payment as explained, as well as the postage for the article - but, you (the person sending the article via COD) must pay the given fees to the postal service for the privilege of doing business thusly.
On arrival of the sent article, if the carrier is handed cash,the recipient must pay an extra fee, for which the USPS will send the shipper a postal money order in the amount collected. Restricted delivery is available as an option with this service, and your article is insured by default for the amount to be collected.
It's a bit of a pain to deal with this service, though. There is no guarantee that the recipient won't skip out on the item (and leave you holding the bag), so it's a risk you take. Many businesses won't do COD anymore as such, and the fees tell you why - it's pricey, and repeated skipped CODs will wind up costing you a bundle.
The cost of this service is variable. The base fee is $4.50 if the amount to collect is $.01-$50, $5.50 for $50.01-$100, and $1 for each increment of $100, up to $1000. In addition, a notice of nondelivery costs $3 (a slip of paper stating your item was returned to you), to alter your COD costs $3, and Registered COD (COD on an item of registered mail) simply costs $4 regardless of the amount.
Update, 11Jul2004 - Seems the mail carrier I use will accept cash, plus the money order fee. Updated accordingly.