Rent, salaries, software subscriptions, agency retainers, insurance. They do not move with the number of orders, which is what makes them fixed, and it is also what makes them dangerous: they are still there in the month nothing sells. They grow by renewal rather than by decision, which is why a quarterly fixed cost review belongs in the calendar.
They should be spread across the days they cover rather than landing whole on the day the invoice arrives, otherwise one day a month looks catastrophic and the rest look better than they are. Even spread, a slow day can show a loss, and a negative margin on one day is rarely a bug.
A yearly invoice divided by 365 and a monthly one multiplied by twelve and divided by 365 both give you a daily slice, which is what a daily P&L needs.
How much you have to sell before they are covered is the question an ecommerce break-even calculator answers.
