How to calculate your Black Friday discount from your margin

Key takeaways

Contribution per order = price received − product cost − fees and logistics − acquisition cost; if the result is negative, every sale increases the loss.
A 20% discount on the price can cut the contribution by 60% or more, because costs do not fall along with the price.
Procon-SP, the São Paulo consumer protection agency (2025), recommends using the lowest price charged in the last 60 days as the reference, to avoid "half of double" fake markdowns.
Free gifts, free shipping and installments have real costs and go into the math as a disguised discount.
Every agent needs to know the floor and their authority: how far they can negotiate alone and when they need approval.

What is contribution margin and why does it decide the discount?

Contribution margin is what is left from each order after paying for everything that only exists because that order existed: product, payment fee, packaging, subsidized shipping, commission and acquisition cost. Fixed expenses and profit come out of it. A discount that zeroes out the contribution turns revenue into losses with more work.

The per-order math is this:

Contribution = price received − product cost − fees and logistics − acquisition cost

Two details matter on Black Friday. Proportional fees (card, gateway) fall along with the price; fixed costs per order (packaging, shipping) do not, so recalculate every line. And the acquisition cost (ad spend ÷ paid sales) tends to rise in November, because Meta's auction gets more competitive; compare it with your own account's previous November, not with market averages.

How to calculate the maximum discount (step-by-step hypothetical example)

The example below is hypothetical; the reasoning works for a product, a service or a digital product.

A store sells a kit for R$ 200 on WhatsApp and wants to know how low it can go on Black Friday.

Variable costs at full price: product R$ 90; payment link fee 4% = R$ 8; packaging and subsidized shipping R$ 12; acquisition cost R$ 30 (R$ 1,500 in ads for the month ÷ 50 paid sales). Total: R$ 140.
Contribution at full price: 200 − 140 = R$ 60 per order (30% of the price).
Contribution floor: the owner decides she needs at least R$ 25 per order to cover fixed expenses and the risk of cancellation.
Price floor: the payment fee changes with the price, so solve: price − 0.04 × price − 90 − 12 − 30 = 25 → 0.96 × price = 157 → minimum price ≈ R$ 163.55. Round to R$ 164.
Maximum discount: (200 − 164) ÷ 200 = 18%.
Testing the "market" discount: with 30% off, the price drops to R$ 140; contribution = 140 − 5.60 − 90 − 12 − 30 = R$ 2.40. The store would need to sell 25 kits to earn what it used to earn from one.

Decision rule: if the discount the market is offering falls below your price floor, do not follow it; change the offer (bundle, benefit, payment terms) or pick another product for Black Friday. The article on how to build a clear offer shows how to present that to the customer.

Discount, bundle, free gift or free shipping: which costs less?

The effective cost of each format is different, and what looks "free" to the customer is never free for the business.

Format	Effective cost for the business	When to use	When to avoid
Direct discount (e.g., 18%)	100% of the discount comes out of the contribution	High-margin product with a comparable price	Tight margin or high proportional fees
Bundle (2 products for a lower price)	Cost of the second product minus what it would already earn	Moving stalled inventory and raising average order value	When the second item has a negative margin
Free gift	Cost of the gift + packaging + weight in shipping	Gift that is cheap to produce and expensive to buy	Gift the customer does not value
Free shipping	Average shipping cost per order	High order value and relatively low shipping	Heavy product or distant customer

Decision rule: if the contribution at full price is below 25% of the price, prefer a bundle or a service benefit (faster delivery, support, extended warranty) over a direct discount. A service benefit costs team time; a direct discount costs new money.

How to set the floor and the negotiation authority?

Define two numbers per product, in writing: the floor (the price below which nobody sells) and the authority (how far the agent can negotiate without approval).

Floor: calculated as in the example above, per product. Products with different cost structures have different floors.
Agent authority: for example, up to 5% below the advertised offer, to close a hesitant customer.
Manager authority: from the agent's limit down to the floor.
Below the floor: only the owner, and only with a recorded justification (e.g., inventory about to expire).

This prevents the most expensive Black Friday mistake on WhatsApp: every salesperson invents a discount, the customer compares two conversations and gets two prices. The business is stuck with the lower one, because under the Brazilian Consumer Protection Code (CDC, art. 30) any sufficiently precise offer binds the supplier. A CRM with quick replies and per-lead history, such as RASTRO, helps keep the same offer text in every conversation and record who authorized each exception.

Which reference price to use to avoid "half of double"?

Use the lowest price charged in the last 60 days as the "was/now" reference. That is the recommendation of Procon-SP (2025), which inspected 533 physical stores in November 2025 and found irregularities in 128 (24%), most of them pricing-related.

"Half of double" is the practice of raising the price in the preceding weeks to advertise a bigger discount. Besides being misleading advertising (CDC, art. 37), it destroys trust: 7.26% of Procon-SP complaints (2025) were for unfulfilled offers or misleading advertising, and Reclame Aqui (2025) logged 10.7% for misleading advertising.

Practical rules: if you raised prices in October, the reference is the lowest price of the previous 60 days, not the new one; if the product is new, advertise without a "was/now"; and keep reports of the price charged by date, because the burden of proof is yours.

Which common mistakes destroy margin on Black Friday?

The five most common mistakes:

Copying the competitor's percentage. They may have a product cost 40% lower or be burning through inventory. Your discount comes out of your contribution.
Forgetting the acquisition cost. A discount calculated with September's cost per sale is wrong in November, when Meta's auction is more competitive.
Applying a single percentage across the catalog. A product with a 50% margin can take a 30% discount; another with a 20% margin breaks at 15%.
Ignoring the smaller average order value. According to Confi Neotrust (2025), the Black Friday average order value fell even with more orders; smaller orders raise packaging and shipping per real sold.
Negotiating without a floor in the follow-up. See follow-up without pressuring the customer: the problem is not picking the conversation back up, it is giving a new discount with every message.

What to measure to know whether the discount paid off?

Measure total contribution after cancellations, not revenue. More orders is only better if the additional contribution covers the additional cost.

Metric	Formula	Good	Bad
Contribution per order	price received − variable costs − acquisition cost	above the defined floor	below the floor in more than 10% of orders
Net total contribution	sum of contributions from paid orders − cost of canceled ones	higher than a normal month	lower than a normal month with more work
Cost per sale	ad spend ÷ paid sales	below 50% of the contribution at full price	close to or above the contribution
Exception rate	orders below the agent's authority ÷ total orders	less than 10%	more than 25% (the advertised offer is not convincing)

Cost per sale per ad requires knowing which ad originated each paid sale on WhatsApp; without source tracking, only the account average exists. See dashboard: what the business owner should look at.

Frequently asked questions

Sources

How much discount should I give on Black Friday?

The most that keeps the contribution per order above your floor. Calculate price received minus variable costs and acquisition cost; the discount is the gap between the full price and the minimum price that still covers the floor. The percentage varies by product, because each one has a different margin.

Does the biggest discount guarantee more sales?

No. Availability, delivery time, trust in the store and ease of payment weigh as much as price. According to Opinion Box (2025), 60% of Brazilians have already bought through WhatsApp, a channel that sells on convenience and service, not only on price. Test a sustainable offer before following the competitor.

Can I use the manufacturer's "was" price as a reference?

Yes, as long as you state that it is the manufacturer's suggested price and it actually exists. For products you have sold in the last 60 days, the safe reference is the lowest price you yourself charged, as recommended by Procon-SP (2025).

How do I handle a customer asking for more discount on WhatsApp?

With defined authority. The agent offers up to their limit; above that, they check with the manager; below the floor, they decline politely and offer an alternative (bundle, installments, delivery time). A new discount with every message teaches the customer to wait for the next one.