Fix the inaccuracy at the source rather than in Claude. Cost per item sits on the variant (Products > open product > Variants > Cost per item) and is what Shopify’s own profit reporting reads, so bulk-fix it via the product CSV export/import using the Cost per item column. Most MCP tools don’t return that field, which is why it looks wrong even when it’s filled in correctly.
Then stop feeding Claude numbers. Give it the Shopify order and payout exports plus your ad platform CSV with matching date and order ID columns, and have it write the join (a script or spreadsheet formula) instead of pasting figures into chat. Keep the same column layout every week so only the window changes.
Shipping bought outside Shopify stays manual; that’s one column, not the whole model.