You don't actually pay taxes and duty on a GSP shipment. Pitney Bowes does that on your behalf and is on record as having paid those when your item is imported.
The "import charges" on a GSP-forwarded item are an estimate of the taxes and duty that would be applied to the item plus any processing fees associated with it.
You may have seen a post or two suggesting that the "import charges" can be written off as a business expense. I'd check with your accountant to make sure that's acceptable to CRA. I can't see why they wouldn't be, but it's good to check with a professional.