It is written into the new rules that the "transportation carrier" is the one who will be responsible for collecting and paying the customs duties and fees. Also, the USPS has announced that they will no longer accept any packages which arrive without customs already paid (a shipping method known as DDP...Delivery Duty Paid.)

Common carriers like UPS and FedEx are used to offering this kind of service, and have the tools to execute it. It is a service where the seller collects all necessary duties and fees at the time of sale, then hands them over to their shipping service, and that service then pays US Customs.

While common carriers can operate this way, and have, no nation's postal services are set up to. De Minimis was in effect so long that none have the ability to calculate, collect and pay the fees up front before handing the packages over to the USPS, and USPS will not accept the package any other way.

Some postal services are working on getting set up to operate this way. DHL's statement that they expect the situation to be temporary is sort of a clue that they probably plan to have a Delivery Duty Paid system set up so that they can then hand off the package to the USPS. Their common carrier service already has this ability. They only need to modify it for this new postal requirement.

Common carriers can operate two ways...this Delivery Duty Paid method, where the seller collects the fees from the buyer up front before shipping...or with Delivery Duty Unpaid (DDU) which is the method you are describing, where the seller just ships it, then all the fees are collected at the other end. While postal shipments may no longer operate like this, if the shipment is handled entirely within the common carrier's network (in other words not handed off to USPS at any point. Delivered "door to door" by the common carrier) then DDU can still be used. But, because common carriers use customs brokers, and usually pay the customs, then collect upon delivery, their fees for this service are significantly higher than postal rates.

Your postal service operates how the USPS used to. USPS used to accept non-de minimis packages which required a tariff to be paid. USPS would get the package from US Customs with the paperwork and amounts due shown, then collect them upon delivery to the customer. USPS will no longer do this. They will not accept any package that is not already prepaid. So, any service that wants to hand off to USPS (which is every other nation's postal system) must now somehow arrange to have the duty already paid.

Hopes this answers it for you.