[ryanmercer]

>surely packages should go through customs before entering a country?

They do. I don't think you realize just how many packages enter the United States on a daily basis though and that Customs only physically inspects a very tiny portion of those packages.

Each year, more than 11 million maritime containers arrive at our seaports. At land borders, another 11 million arrive by truck and 2.7 million by rail. Each of those containers could contain 1 shipment between 1 shipper and 1 importer of record or it could contain dozens or hundreds of individual shipments from multiple shippers to multiple importers of record.

Then you see tens of thousands of packages enter the United States daily via air.

The United States has 328 ports of entry in the country and 13 pre-clearance offices in other countries. In 2018 the United States imported $3.1 trillion and exported $2.5 trillion in goods and services.