Within ecomdash, there is a storefront fulfillment indicator to define whether the product is going to be 'Fulfilled by Merchant' or 'Fulfilled by Amazon'.
1. Navigate to Inventory -> Inventory Management
2. Click the binoculars icon to view the product details of your SKU.
3. Choose the 'Storefront' tab.
4. You will see a fulfillment column for each storefront. This is where you can set the fulfillment option at the product level.