AHA!!
According to the Shippo spokesperson during the recent discussion-- we can use Shippo to print a USPS Return Shipping Label.
To answer your USPS return label question, which is related to carrier accounts, Shippo does come with master accounts immediately upon sign up. USPS and DHL Express (for US originated shipments) are two of them.
For returns from the US to Canada, you’ll need to manually input the order information.
1) Take note of your customer address and package information. On the Orders page, head to "Create Order"
2) Input your customer's address as the "Sender Address", and input your return location as the "Recipient Address"
3) Input your package information, customs details, and select from the available USPS or DHL Express rates.
4) Purchase label, download, and send to the customer.
You'll be charged for the label as usual in your invoice. If your customer does not use the label, you will receive a refund after 30 days – which also voids the label.
Since I have so far been unable to print a Canada Post Shippo label, and rarely use labels anyway, I can't confirm this information.
