One problem is that Google needs to be able to access pages showing the right currency for each country. You can only advertise in countries if your feed sends people to pages showing the local currency. As far as I can tell, in your case, Shopify will only provide a feed using URLs that result in USD prices.
Andy, use DataFeedWatch. Shopify's Google Shopping app just doesn't cut it for a shopping campaign of decent size due to the lack of customization available. My AdWords clients are put on DataFeedWatch usually when they have 50+ products or the feed should have more customization to drive down costs and increase relevancy.
Having said that, like what was mentioned, the landing page needs to correspond with the currency of the country. How this is done depends on your geographic and currency setup with your theme and any apps used.
Managing multiple Google Merchant Center Feeds is one of the reasons we built ShoppingFeeder. You can use us to generate feeds for Merchant Center including the soon to be launched countries of South Africa, Ireland, Singapore and New Zealand and we also do automatic currency conversion for you if you don't have the specified currency in your store.
Always happy to answer any questions you may have
I am getting literally crazy! I have the same problem in 5 stores, I'm using DataFeedWatch but I got this problem (please, see the picture attached). In a nutshell, my ads get published in UK like: £price + tax ($price) . This is weird because:
-there is no tax in UK
-it gets published even the dollar price.
The price on the website are in dollar, we sell worldwide and the prices (user's side) change according where the users access to. The Default currency on the Merchant center is in US dollar.