A 20% Discount Does Not Reduce Your Profit by 20%.
Discounts reduce the selling price immediately. Most product, fulfillment and operating costs do not fall with it. That is why a promotion that looks modest on the storefront can create a much larger reduction in contribution underneath the sale.
Discounts act on revenue. Your costs often stay exactly where they were.
A 20% promotion sounds straightforward. Take a $100 product, remove $20 from the selling price and sell it for $80.
From the customer’s perspective, that is exactly what happened. The price fell by one fifth.
But profitability does not begin at $100 and fall proportionally with price. The business still has to pay for the product, transaction costs, fulfillment, shipping and other costs attached to the sale.
If those costs remain largely unchanged, every dollar of discount comes primarily out of the economic value that would otherwise have remained.
This $100 product normally contributes $40.
We will use a simplified $60 variable cost base so the effect of discounting is easy to isolate.
The price declines gradually. Contribution collapses much faster.
Measure the discount against contribution, not just against price.
Before the promotion, the business keeps $40 of contribution.
The 20% discount removes $20 from the selling price. Because the $60 cost base remains unchanged, the entire $20 reduction comes out of contribution.
That means the relevant comparison is not $20 against the original $100 selling price.
It is $20 against the original $40 contribution.
How many more units must the 20% discount sell?
If contribution per unit falls from $40 to $20, the business needs twice as many units merely to generate the same total contribution.
Deeper discounts require disproportionately larger volume increases.
With the same $60 variable cost base, each promotion depth creates a different break-even requirement.
Margin sacrifice can be rational — when it buys something valuable.
The objective is not to eliminate promotions. It is to know what economic outcome the promotion is expected to create.
Lower first-order contribution can make sense if future purchases generate enough retained profit to justify the acquisition cost.
A promotion tied to a minimum spend may encourage additional product contribution rather than discounting an unchanged basket.
Clearing slow-moving stock can release cash and reduce inventory risk even when the immediate unit margin is weaker.
The uplift must be measured against the new contribution per unit, not simply against the increase in gross orders or revenue.
Selective offers can support retention, but permanent discount dependence can train customers to avoid full-price purchasing.
The closer price gets to variable cost, the more violent the margin compression becomes.
Every additional discount dollar consumes a larger share of the contribution that remains.
A promotion can produce more orders and more revenue while still generating less total economic value. That happens whenever the increase in volume is too small to compensate for the weaker contribution created by each discounted sale.
That is why promotion performance should be judged against a contribution baseline. The question is not merely whether customers responded, but whether their response paid for the margin sacrificed.
A discount can be small commercially and enormous economically.
A 20% discount reduced the selling price to $80 while the $60 variable cost base remained unchanged. Contribution therefore fell to $20.
The storefront showed 20% off. The economics showed 50% less contribution per unit — meaning unit volume had to double merely to break even.
Promotions should be measured by the profit they preserve — not only by the sales they create.
MarginLab helps ecommerce operators investigate discount exposure, product economics and margin deterioration behind store performance.