Nit-picking time: The GSP is technically no more. It's been replaced by a similar program that's been confusingly called "eBay International Shipping" (eIS).
eIS has a lot of perks for the sellers who use it. It serves a lot of countries in addition to Canada, and some buyers don't seem to mind it too much if it bypasses the difficult process of a direct mailed shipment going through customs in their particular country. But for us Canadians who have seen a lot of the impediments to trade with the US whittled away since the end of the Second World War, the only way to make it work better than direct shipments would likely be to use expensive express shipping and handling which nobody in their right mind would pay for.
The GSP and eIS have made many Canadian eBayers aware that their low-value casual imports are just as subject to charges for taxes (HST/GST/PST) as their purchases from Canadian sellers. I don't know how long your "long time" on eBay is, but when my wife and I first joined eBay, we could count on about half of our direct mailed purchases getting assessed tax (and occasionally duty) charges by Canada Border Services that we'd have to pay before getting our item from the post office or mail carrier. By the late '00s, seeing those charges on those purchases was a very rare thing; CBSA was giving most of them a pass even though they were technically valued at more than the tax and duty-free limit of C$20.
Items sent through GSP and eIS go through customs pre-clearance processes that may not be immediately obvious, but they go through customs processing regardless. You're not being charged for something that isn't being done. The confusing part seems to be that right now, items handled by eIS can be charged taxes at checkout or they may be charged upon receipt of the item, depending on the nature of the item and/or its value. At some point, the system is supposed to be modified so that buyers can choose how they want the taxes handled, but eIS seems to be going through a lot of growing pains that are slowing down the rollout of new features, particularly the ability for the program to combine shipments of multiple items from the same seller.
