Global NA Account, how to ship to USA and Canada ?

Need Guidance :

I recently sent out my first inventory via AGL (Amazon Global Logistics), and it’s now distributed across nearly 58 warehouses. My active listings have also been copied to Canada and Mexico, allowing customers in those countries to purchase items if they pay the import duties. However, lowering prices to stay competitive would leave me with negligible margins

I am considering setting up my Shopify store for Canada and the USA. I tested placing an order with MCF (Multi-Channel Fulfillment), but it failed and gave an error. I discovered that only media and books can be fulfilled via MCF to Canada.

I have created a new product listing in the US marketplace and want to send my new shipment from overseas to both the US and Canada. I can only replenish the inventory created in the US marketplace, which isn’t active yet as I’m preparing to ship it. I haven’t found a way to send inventory to Canada to ensure my Amazon Canada store has stock for MCF. I can’t find the UPC or ASIN on the Canada side either.What should I do to send my inventory to both the US and Canada?