Thanks for the information you provided.(注意应该是Provided,过去时）
My order has arrived in China.
As we konw,this order was delivered by UPS shipping.Due to UPS' active customs clearance,this order was inspected by Chinese Customs.We now need to pay USD 100 as importing duty.
Clearly,it won't be inspected by the Customs if shipped by USPS.
Due to your mistake,i think you should be responsible for the 100$ duty.
其实翻译大同小异,只要让对方明白个中缘由才行,赔偿了您这100美金才是正事儿.强调他们的错误,把责任退给他们.必要时电话打过去!
希望您顺利解决此事儿!