As I recall, normally when sending packages internationally, one has to declare what an item is and its value.
So that’ll mean that customs knows how much is involved.

kagis

https://www.usps.com/international/customs-forms.htm

Customs Forms

Filling Out Customs Forms Online

When you ship items from the U.S. to another country, you must fill out customs forms (except for First-Class Mail International® letters and large envelopes under 15.994 oz):

• The form you need depends on the USPS® mail service you use (and the total value of what you send).
• You can print just a customs form or use Click-N-Ship® service to pay for postage and print an international shipping label and a customs form.

kagis

Sounds like at least USPS can do a “receiver-pays” service.

https://www.cbp.gov/trade/basic-import-export/internet-purchases

Importing Process Paying Duty: The importer is ultimately responsible for paying any duty owed on an import. Determining duty can be very complicated, and while shipping services will often give an estimate for what the duty rate on an item might be, only CBP can make a final determination about what is owed. You should not be misled into thinking your purchase price includes duty because the seller cannot say with absolute certainty what the duty will be. As a rule, a purchase price that includes shipping and handling does not include duty or any costs associated with clearing the goods through CBP. First time importers are often surprised by bills they receive for duty, U.S. Customs and Border Protection merchandise processing fee, and something referred to as “customs fees,” which are actually charges for the services of the broker who cleared your goods through CBP.

How you pay duty depends on how your goods were shipped. If your goods were shipped through the International Postal Service, you will need to pay the mail carrier and/or go to your local post office to pay any duty and processing fees owed when your package arrives at that post office. If your goods were sent by a courier service, that service will either bill you for the duty they paid on your behalf or require payment on delivery.

IIRC, there’s a de minimis exception, where one doesn’t have to pay fees on items below a certain value in a single shipment. There was some controversy over this, as it meant that someone who did bulk imports from China to the US and sold things via, say, Amazon would have to pay tariffs, but someone who imported a single unit via AliBaba or something wouldn’t. This had been giving AliBaba a benefit, because it could sell tariff-free. And there was discussion about revising this, to people selling via AliBaba and Amazon.

kagis

Yeah:

https://www.yondatax.com/blog/how-to-handle-us-china-tariffs-and-the-end-of-de-minimis

And while the president has been relatively quiet on the topic of de minimis—the long-standing policy that allows goods under $800 to enter the U.S. without duty—new guidance from the White House signals big change. As of early April, de minimis exemptions for goods from China are ending.

Starting May 2, 2025, the U.S. is ending de minimis exemptions for goods coming from China and Hong Kong.

Right now, the de minimis rule—under 19 U.S.C. § 1321(a)(2)(C)—lets one shipment per person, per day, valued at $800 or less enter the U.S. without paying duty or import tax.

That benefit is going away specifically for China and Hong Kong. For now, it will still apply to other countries—but that could change in the future.

EDIT: Tariffs in general aren’t new, though. Like, Trump increased tariffs, but we always had tariffs on things. So the infrastructure will already be in place.