Unfortunately, the short answer is no. Shopify requires that each of your products are assigned a fulfillment method during initial setup. And then that products are assigned the correct shipping rate within your shipping profiles.
So, in order for our app to fulfill orders the products would need to be set to FBA Shipping and in the FBA Shipping profile before the customers place orders. There is not a way to change the fulfillment service after the order has been placed.
This is what the Products Bulk editor looks like. Where you can change your FBA products ‘Stocked At’ to FBA Shipping. So the store must decide at the product level if our app will be involved or not. This happens before the orders are placed. As Shopify does not allow you to assign a fulfillment service to orders after they are placed.
And this is how you get to the bulk editor in Shopify