How to select “Fulfillment Location” and why is it important?
Before selecting Fulfillment location, Make Sure locations must be enabled in Shopify
Once you add and enable the location from Shopify Setting then the added location will be visible in the App setting under Configuration page of the App.
If you have more than one active location, you need to specify the priority sequence for order fulfillment.
When the orders are placed through any online sales channel, they are assigned to a location where they should be fulfilled based on the list and available inventory.
If any location that is selected in the Fulfillment location under our App setting can fulfill the entire order, then the inventory is drawn from that location and will update the tracking information.
If you don't select Fulfillment Location then
** Inventory will not sync
** Tracking & Shipment information on Shopify will not update as per the AMAZON Shipment information.
Refer Shopify Link for more info