The import charges are considered estimates and not finalized until the buyer goes through Checkout. Buyers who have gone through the “offer” process on a GSP-forwarded item have found that they get sorted out when they pay for the item.
Basically, when you see the item on the listing page, the import charge estimate is based on your postal code. The shipping location you use in your searches affects what the calculator bot bases the tax (GST or HST) portion on. In the offer process, the bot has no postal code to immediately refer to so it calculates the tax portion based on the highest GST rate in Canada (15%). If the item is subject to duty, the amount estimated for duty will also vary with the amount estimated for tax, so there’s also that.
(Just guessing here, but I suspect the shipping charge could also change once your shipping address is finalized, so keep that in mind.)
If you accept the offer and go to checkout, once you confirm your shipping address, the calculator bot will have your postal code and will be able to finalize the import charges. This is my understanding from a couple of buyers who have posted here in the past about their experiences with the offer process.
By the way, spotting GSP-forwarded items in search results is actually pretty straightforward. Just watch out for items where an amount for a “shipping estimate” instead of just “shipping” is given.
