Seeing as there's a 10% difference between .com and .ca, I tried plugging some things into my cart to see what kind of value you could get from Prime on .com. I highly doubt they give free shipping for Canadians, at best there might be faster handling (and the cheapest rate can take over a week...).
First up, Ys 8:
$60 (USD, all prices are in USD and without discounts)
+$7 Shipping
+$7.41 import fees (that's a touch over 12%, the tax rate in BC. It's prettymuch a wash)
Not that great a value, paying more than 10% for shipping means it's a loss. I tried doubling the quantity though:
$120
+$9 Shipping
Shipping didn't go up much, that's a savings right there. I tried increasing the quantity, but removing Ys and adding .hack//G.U. instead:
$150
+$6 shipping
The shipping price went down with 3 copies of G.U....? I forgot Ys 8 has a fancy day-1 edition, so I guess they're getting some sort of actual shipping price.
Next up I went crazy and added 12 PS4 games, many of which aren't on Amazon.ca or simply don't have *any* discount:
$560
+$16 shipping
Holy crap, that's $1.33 shipping per game. There's definitely some value there. I don't know how they'd ship them though, it'd be silly if they hold them till they're all ready...
The .com Prime subscription prices are different too. It's $100 for a year, maybe because Amazon Video isn't worthless down there? Also you can buy a single month of Prime for $11. So it might be worthwhile to occasionally subscribe for a month and throw down a mega order. Although I'm also unsure if .com works like it does up here, you may need a Prime subscription when it ships, both sites have always stated that's required.