I working on a 66 Honda so I've been getting lots of stuff shipped up, all USPS. Package contents labeled around $20, $50, $100 and one @ over $200.
The only duty I've been dinged with is one of the packages around the $50 mark. All the others came to me clean and clear.
I'm sure there is some reasoning to why some packages make it across free and others get dinged but seems like a total crap shoot to me with USPS.

If you do get something shipped by a shipping company - UPS, FED-EX etc. You do pay the shipping plus their ridiculous brokerage fees.