For my car restoration all parts were imported from UK. Some parts came via Royal Mail (equivalent to USPS) and others via courier. USPS will be a fraction of the cost compared to courier. Even between couriers there is a huge price difference. All of my items sent by either by courier or Royal Mail arrived safely. So we can assume items sent by USPS will arrive safely. In a few cases there was no customs duty charged, the package just reached my house. In other cases duty was charged. Duty is charged on the landed value (price of goods + postage). Items sent via USPS will have a lower postage cost, will be charged a lower duty amount.
On my last consignment, arriving from UK sent though UPS courier here is the breakdown:
Parts cost = GBP 90/-
Postage cost via courier - UPS = GBP 75/-
Total landed price = GBP 165/-
Rupee equivalent = @ Rs 170/GBP = 28,050/-
Custom duty charged = Rs 10,040.
So speaking very roughly we can say the duty is about 35%-40% of the landed price.
Hope this helps.