My Application is mostly US based, I do have some Canadian users that are wanting to take advantage of using Square for the Point of Sale. I am running into an issue if I have change the currency I get bad request errors if I include an app_fee. Maybe its on a later release from what I have. Just checking in to see whats available.
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Additional Documentation
Collect Application Fees
Guidelines and Limitations
Payment API: Process Online and In-Person Payments
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At this time you can only collect app_fee_money in the currency of your Square account. So if you have a USD account, you’ll only be able to collect app_fee_money in USD. If you’d like to collect app fees in CAD you’ll need to have a valid CAD Square account connected to a Canadian bank with payment processing enabled.
Also collecting app_fee_money with the Terminal API isn’t currently supported outside of the US at this time. 
Thanks for the reply. Looking forward to this feature in the future. I did see a way to auto invoice a customer. Looks like a viable solution for my users outside the US.