The test is a question rather than a category: would this cost exist if the order had not happened? Rent, salaries, software subscriptions and the accountant all fail it, which is what makes them the fixed block that contribution has to cover.
Some costs are variable per order rather than per euro, and the distinction decides how a falling basket size hurts. The parcel, the picking and the fixed leg of the payment fee cost the same on a 20 euro order as on a 200 euro one, so they quietly destroy margin whenever average order value falls.
Advertising sits at the edge of the definition on purpose. A committed monthly budget behaves like a fixed cost; spend that scales with the orders it wins behaves like a variable one. Both readings are defensible, and they give different break-even points, so a statement should say which one it used.
