When I see a comic that I am interested in buying but the seller uses the GSP, I hit the back button immediately.
In my experience with the GSP, once I've overpaid on the two shipping charges, I've had this GSP "Shipping Hub" in Kentucky send me a box of comics to me in Canada where it is dropped off on my sundeck in the pouring rain with no signature required, nobody home. I've also had items arrive damaged using the GSP.
All other non-GSP shipments arrive through Canada Post at my local Post Office where all my incoming comic book purchases should arrive.
So I'm done with the GSP as a Canadian buyer, simply not worth it for me.