That's on the importer side for delivery to you.

As I understand it, US customs wants foreign carriers to collect tarriffs when packages are shipped, and pay them to the US.

There is no system to do this, nor a system to actually receive payments and associate them with a package. Nor any clarity on what the rules actually are and thus what the import duties will be when things arrive.

The normal course of things is that things get shipped, hit customs and get assessed for duties, and then the importer pays for release. If you've ever experienced differently it's because someone is handling it for you - e.g. Amazon provide this service and absorb the complexity and risk.