Quote: "collected by the post office at delivery."

No, the mail carrier is not going to collect this from you before he delivers your package.

Foreign mail carrier services, be they the actual foreign postal entities, or carriers contracted by them, must remit the customs payment to CBP at the time they enter the shipment into the country.

Those carriers collect the money from the overseas shippers as part of what they charge them to ship it to the US. And those overseas shippers...meaning the overseas business you bought it from who then hand it to their postal service...collect the money from the buyer in either the price of the item itself, or more likely, as part of the shipping charge they bill you for.

So, you get billed. The business pays it to their shipper. Their shipper hands it to CBP upon entering the shipment into the US.

If an item shows up unpaid for by the shipper, or with an incorrect amount paid (CBP checks each item on the manifest with the payment they receive) CBP hands it over to an authorized, bonded collections broker, who pays CBP for it, warehouses it, then goes and bills the receiver (you) for it.

So, if your package enters the US without the duty already paid, you will not be bothered by either CBP or your mail carrier. You will be bothered by an authorized, bonded customs collection agency, who will only turn the package over to you after you pay everything, including their warehouse storage charges!

If you want to look it up, it is procedures for DDP (Delivery Duty Paid) and DDU (Delivery Duty Unpaid) in the operations of CBP (Customs Border Protection).