I was contacted by a couple of fans who told me that they are not able to order Season 6 of Heartland from Amazon Canada as non-Canadian residents. So I contacted Amazon about this and it sounds like this is an issue to be solved soon.
This is the message I got:
"Thanks for writing back to us with the information we requested.
I checked and confirmed that there is an ongoing issue in shipping the preorder
item "Heartland - Complete Season 6 / Heartland - Saison 6 (Bilingual)" to
international address.
We've reported this to our technical team, and they're working on taking care of
it. Once the issue is corrected you could place the order.
You can place the order when the item gets released to avoid further
complications."
So I hope all fans will be able to enjoy watching Season 6 when it is released in October :)