60-second answer
Use net sales per order as the starting point.
For the ROAS Break model, start with Shopify net sales for a mature period, divide by the corresponding order count, and exclude tax. Reconcile shipping revenue separately. Then add landed product cost, fulfillment, payment or marketplace fees, return handling, and any other variable order cost not already reflected in net sales.
Model AOV = mature net product sales / corresponding orders
Do not double-count: If returns have already reduced net sales, do not enter the same refunded revenue again as a returns allowance. Add only economic losses that remain outside the sales figure.