Why am I being asked to pay a tax when I already paid? (International Only)

Any additional customs or import duties charged are out of our control.  In fact, each country determines not only the rates, but when they are going to impose them (we are told this can fluctuate greatly!)  In order to be proactive however and assist our international customers with this potential issue, we underestimate the cost of the declared goods for shipping purposes in case such surcharges occur in order to lower the amount considerably.