I just activated Shopify's built-in Fulfillment by Amazon integration. When I request for an order to be fulfilled, I get an error saying "Value for parameter SellerId is invalid. Reason: InvalidValue". See screenshot. As far as I know, the SellerID is sent automatically by Shopify to Amazon's API.
I have a US/Canada registered Amazon Seller Central account and sell in USD on Shopify. Has anyone else had this issue and resolved it?
Shopify's built-in FBA integration doesn't always work as advertised. If you want to make sure your orders will go through to Amazon, I would use an app like FBA Shipping.
But if you want to stick with Shopify's integration, I would make sure that Shopify is authorized on your Amazon account. Here's a video that shows you how to do that. And you can double check your Seller ID by following the steps in this video.