In your own shop there is no marketplace commission. The costs per sale arise as a payment fee, the share the payment provider keeps for processing.
What determines the amount?
The payment method. Credit card, PayPal, Klarna, Sofort and invoice each settle differently, usually as a percentage plus a fixed amount per transaction. The difference between the cheapest and the most expensive payment method in your own shop is often greater than the difference between two marketplace commissions.
For your calculation this means: the basket value alone says nothing about the cost of the sale. Only the payment method chosen turns it into a figure.
What happens with a refund?
The payment fee is usually retained. A refunded order therefore costs the fee even though no revenue is left, the same effect as a return on a marketplace, just in a different place.
Does that make your own shop cheaper?
On costs per sale, usually yes, because no commission applies. In return, the shop carries visibility itself: advertising, content and reach replace the customers a marketplace brings with it. In the profit calculation these costs do not appear per order but as running expenses, which is why a comparison only makes sense across the contribution margin and the indirect costs together.
Payment fees per order
The profitkonsole for Shopify takes the fees from your payout data and calculates payment methods for which no statement is available using the rates you have entered.
Contribution margin, not revenue
Every sales channel in one dashboard: marketplace data and your own costs brought together, per item and per channel.