Why free shipping has shipping charge on my order?

There are a few customers sent us an email " Why I have to pay "shipping fee"on Free Shipping Order in U.S? "

We would like to let you understand the invoice that the mailman give to you isn`t shipping fee,it is custom fee or tax,charged for U.S custom. Because the US customs are very strict with import packages sometimes, most of packages exist custom fee or tax are charged from U.S citizens, You know that is out of ours and shipping company`s control .Since it is US country policy .If there is any inconveience ,pls forgive us ,also appreciate your understanding!