A price that covers the cost and leaves something

A margin is a share of the price, not a mark-up on the cost. Mixing those two up is the most common way a small seller works for free.

What it costs you to make and deliver

What you want to keep, percent of the price

Fees on the sale, percent

Every line above comes out of the boxes you typed. Nothing is stored, nothing is sent, and there is no account behind this page.

If the denominator reaches zero - want plus fees at or above 100 - there is no price. That is the page telling you the wish is too big, not a bug.
Your time is a cost. If it is not in the number you typed, the 'margin' is paying for the hours you did not count.