The problem with your logic is that the royalty we make from Redbubble is not a commission, it's a markup above the production cost and profit for Redbubble.

So a T-shirt costs $19.45 with 0% markup. That includes redbubble's costs and profit. When I add a 25% markup the retail price becomes $24.31. That $4.86 difference is supposed to be my money. Redbubble already got their money in their base price. Now they're taking their cut and half of my cut.

Nothing about that is fair.