Set your FBA products fulfillment service to ‘FBA Shipping’ – July Update
In Shopify click on ‘Products’ on the left hand side of the screen. Then, ‘Select All’ of your products and click ‘Edit Products’

On the next screen click on Columna. Then, scroll down to Inventory and click on ‘Stocked At’.

A new column will appear. Ensure that all of your products or SKUs that are in a Amazon warehouses and are FBA have the Fulfillment Service set to FBA Shipping. All non-FBA items can be assigned to ‘Multiple Locations’ or anything other than ‘FBA Shipping’

Just save your changes and you are good to go!
Wait! What if I just want to change one SKU/product listing 1 at a time?
Navigate to Shopify, click on Products. click on the Product title you wish to edit. .

Scroll down to Inventory’ And look at ‘Inventory will be stocked at’ Change this to FBA Shipping. Then, save your changes!

Now you are good to go! Keep in mind the ‘Inventory will be stocked at’ is just used for order fulfillment to pass the orders on to Amazon. If you are wanting to ‘sync’ your FBA inventory please set that up inside of the app. This is also the location you will navigate to in Shopify to ensure your ‘Location’ matches the ‘Location’ in the sync portion of the app.
And last but not least. What if your single product has a variation on this screen?
You will see your Variation selection. Press edit for the correct Variation.

On the next screen scroll down to ‘Inventory’ and once again look at the ‘Inventory will be stocked at’

Make sure it says FBA Shipping, if it is a FBA item that the app is managing. Press Save on this screen. And you are finished with this product.