Shopify fees look tiny per checkout. Marketplaces tax the sale but supply demand.
Own-store wins when you can acquire buyers cheaply enough that Stripe/Shopify processing beats marketplace final value fees — and you can fulfil reliably.
Use the A/B tab in the calculator to compare marketplace fees against the ad % your own store actually needs. If your paid acquisition is 25% of sale, “cheap checkout” is a mirage.