If you really do still owe $50 on the item, it could be because the seller screwed up and sent the item directly to you instead of through the Global Shipping Center to be forwarded to you.
If it really was sent by "International Priority" through the Global Shipping Program, keep in mind that there's three stages to delivery.
1. Delivery from the seller to the Global Shipping Center
2. Delivery from the Global Shipping Center to a receiving hub in Canada
3. Delivery from the Canadian hub to the buyer.
Any one of these can be considered a "final destination" for the item as there's at least three different shippers involved here. Unless the tracking information you've received is really clear about the fact that delivery was attempted at the location you specified for delivery, it may well be that this "delivery" refers to one of the other two stages. How long has it been since the item was shipped out by the seller?