The basic answer is don't order anything that will be delivered by mail until the labour situation at Canada Post is settled.
If the seller is advertising that he will use a courier like UPS, your shipment will be delivered. But you WILL pay duty, tax and $25 or more in 'customs brokerage fees".
If the seller is using the GPS, you WILL pay duty, tax and ~$5USD 'service charge" but GPS/PitneyBowes will be choosing the carrier once the purchase arrives in Canada. They usually use Canada Post, but are known to have contracts with CanPar. There would be no further service charges by the courier to you.