Tariffs should be paid at passing customs in the US. It should be an 'automatic' process charging the buyer.
Potential problems are that either or both seller and shipping company may refuse or want the tariff paid upfront to avoid them holden the candle or receiving the goods back.
Was/is the same in the EU with buying from the UK after Brexit. This was tricky for 2, 3 years but has no all but sorted itself out with the seller delegating to the shipping company who then charge the buyer. I am currently at that about the HEX Ezcan.
The downside for the buyer is that import tariffs are paid over the product, shipping and admin fees. I thus try avoid buy from outside of EU for several reasons. Luckily BMW Europe being EU.