Been buying from UK/EU for nearly 10 yrs and have only been hit with tariff once. What I can recommend is avoid using commerical courier like FedEx, UPS, DHL, etc. and use local postal service, if possible. My working theory is that customs clearing is revenue for commerical couriers, so they make sure this is done if the declared value exceeded the exemption amount. This does not appear to be the case when shipped via local postal service (Royal mail, Deutsche Post, etc.) they seem to let the destination country decides if a duty is assessed. Again, this is my theory, so don't quote me on it.
I believe ~3% (amount over $800) is a good number to use for the duty imposed, if assessed.
Here's the official US CBP link:
https://www.cbp.gov/trade/basic-import-export/internet-purchases
Good luck!