Here's a couple of ideas to get you started. Partly depends if you need to show them now, or after each sale. None of them perfect, but they might give your customer some insight. 1) Create a new shirt, "print screen" during first step where you enter the retail price and Amazon shows your corresponding royalty. Or, do a screen share group login with client to show them that screen. 2) You can show them the royalty calculation page on Merch,
https://merch.amazon.com/resource/201858580. Although my inclination is that showing this to a customer might confuse them more often than not. 3) On the analyze tab, download one day of sales, it will show Revenue (retail price) and Royalty (your profit) on each shirt sold.