I always price at double my cost; why is that not enough when I sell on a platform that takes commission?

Where the doubling rule comes from

In a traditional shop, a trader bought at one price and sold at twice that, and the rule worked because other expenses came out of total sales rather than each item. On a marketplace the picture changes: part of every price goes to the platform before it reaches you, so double is no longer double.

What actually happens

In the last case profit is back to 50, double the cost as you intended. The margin as a share of price is 42.5%, though, because part of the price went to commission.

Using the calculator your way

1. In the Selling-price calculator, choose how you set profit: markup on product cost.
2. Enter the product cost and type 100 as the target markup.
3. Enter the platform commission as the percentage fee.
4. If you pay shipping, add it: a product costing 50 with shipping of 15 and 15% commission comes out at 135.29.

In this mode markup is measured on product cost only; other costs are added as they are and the price is lifted to cover the commission. That matches how you think if you are used to the doubling rule.

Do not confuse a 100% markup with a 100% margin

If you choose margin mode and type 100, the tool refuses with 15% commission, because commission plus margin make 115% of the price and no price can do that. A 100% margin would mean the whole price is profit and cost is zero, which is impossible. Doubling equals a 100% markup, which is only a 50% margin before any fees. To see how the two relate on your own numbers, try the Profit margin calculator.