Aria here, I'm on the Social Care team at Shopify.
You can have individual shipping zones for the US and Canada with free shipping set up and have a third zone for the rest of the world that does not offer free shipping. This guide explains how to set up shipping zones.
To set up free shipping within one of your zones, I'd recommend using price-based rates. This is what it would look like when you set it up correctly:
You can create a discount code for free shipping in your discounts section but there is no option for discounted shipping as such. You can, of course, use the same price-based rate technique to set up flat rate shipping for locations outside of the US and Canada.
You don't need an app for Canada Post shipping calculator, that is included with Shopify Shipping. You can also purchase Canada Post shipping labels directly from the admin when you are fulfilling an order.
Does this make sense? Let me know if there is anything else you're not sure of.
Social Care Guru
I apologize, I can now see I didn't fully answer your question.
If you wanted to ensure a discount was only accessible to subscribers, you could use the Locksmith app. You could then use a discount link, using Locksmith to hide it from anyone who is not in your group of subscribers.
The only type of shipping discount you can have is free shipping. You don't have the option to offer a $15 shipping discount. What you can do is set up a flat rate shipping rate for the rest of the world, excluding Canada and the US but I don't believe the Locksmith app works for shipping rates.
I can send a feature request for you if you can provide a bit more information on why this is important and the difficulty it causes when this feature is not available.
In the meantime, would you consider discounting the product instead of the shipping for International subscribers?
Social Care Guru