I put something in my cart and then went to checkout and the charges show exactly the same as on the listing, ie. no EXTRA amounts have been added.  It was a GSP item.  I didn't complete checkout as I had no intention of buying it.  

Item cost:  $9.95 US

Postage:  $17.95 US

Import charges:  $6.76 US

GST would be $2.79 US so PB is charging an extra $3.97 US on this item.  I can't be bothered trying it on another item so see if they're charging a standard fee.  Someone else can do that.  It's easy enough to find GSP items to experiment.