If you bought from the UK then there is no mistake.....you need to pay the GST and import charges as well as the international shipping charges.
It sounds as though your seller has used the GSP shipping and you have been charged by the seller to send the item to the GSP hub in the UK. You have then been charged the international shipping and GST by Pitney Bowes.
If you post the item number here someone can confirm that these are the charges you are seeing.