The margin starts at the sale price minus the product cost. Advertising and the platform percent you enter are then subtracted. What remains is the contribution on that order. Divide by the sale price for the margin percent.
A $40 sale with a $15 product cost, $8 of advertising, and a 15% platform fee of $6 leaves $11. The margin on the sale price is 27.5%. Raise the advertising and the $11 falls by the same amount.
Returns, payment-processing fees, and shipping are included only when you put them in the cost or advertising fields. It is not a forecast of how many orders will sell.