Most Amazon sellers who also run Shopify have had this exact argument with their CFO: "Amazon ROAS is down, let's cut spend." Then Shopify direct traffic drops the following month and nobody connects the dots. If you want to know how to track the Amazon halo effect on Shopify DTC sales, you have to stop treating the two channels as separate businesses in separate spreadsheets. They're not. They're one shopper, making one decision, across two storefronts.