Ask most SME owners which product or service makes them the most money, and they'll often point to their best seller. But high sales don't necessarily mean high profitability. The product moving the most units is often quietly the one earning the least per sale and because it sells in volume, it can be dragging down overall profit without ever showing up as a problem on the top line.
Why the best seller is often the worst offender
Revenue and profit are not the same number, but they get treated like they are. A product can be your top seller by revenue and still be barely breaking even, or losing money outright, once every real cost attached to selling it is counted.
This usually happens for a predictable reason: Sometimes, a best seller owes its popularity to aggressive pricing, frequent discounting, or paid promotion. These tactics can help increase sales, but they can also reduce the contribution earned from each sale. High volume then multiplies a small loss per unit into a meaningful drag on total profit while a low-volume, high-margin product sits ignored in the background, actually carrying more of the business than anyone realizes.
The real cost stack most owners aren't tracking
The gap between “revenue” and “what we actually kept” comes from costs that rarely make it into a simple sales report:
Cost of goods: the obvious one, and usually the only one tracked closely.
Shipping and fulfillment: especially when it varies by order size or destination, and especially when the business absorbs part of it to stay competitive.
Returns and refunds: a product with a high return rate is quietly more expensive to sell than its listed cost suggests.
Payment processing and marketplace fees: these differ by channel, and are easy to underestimate when a product sells across more than one.
Discounts and promo codes: if a product regularly sells at a reduced price, the margin on paper and the margin in reality are two different numbers.
Advertising spend: when a specific product or channel is actively promoted, that spend is a real cost of selling it, even though it rarely gets allocated back to the product itself.
None of these costs are hidden exactly, they're just scattered across different reports, which is why they never get added up against a single product.
Why the channel matters as much as the product
The same product can have completely different economics depending on where it's sold. A marketplace sale might carry a commission and a fulfillment fee that a direct website sale doesn't. A wholesale order might come with a lower price but no marketing cost attached at all. Looking at a product's margin as one blended number across all channels hides this completely. It can look mediocre overall while actually being excellent on one channel and a loss-maker on another.
How to run a simple margin audit
Start with your top 15–20 products or services by revenue, then add any products with unusually high returns, heavy discounting, weak margins, or significant advertising spend. This is where most of the leaks live, simply because volume magnifies whatever the per-unit economics happen to be.
Break each one out by the channel it sells through, if it sells through more than one marketplace, own website, wholesale, in-person, since the fee structure usually differs.
Add up the full cost stack per unit, per channel: cost of goods, shipping, returns, payment/marketplace fees, average discount applied, and any directly attributable ad spend.
Calculate the contribution margin per unit by subtracting net variable costs from the net selling price. Then, where relevant, deduct attributable advertising and other product or channel-specific costs to calculate a further profitability measure. Keep shared fixed overhead separate unless you have a clear allocation method, and label each measure consistently.
Rank by total dollars contributed (margin per unit multiplied by volume), not just by margin percentage or by revenue alone. A product with a thinner margin but far higher volume can still be worth more to the business than a high-margin product that barely sells. The ranking only makes sense once both numbers are combined.
Flag anything sitting at or below breakeven, especially the higher-volume items. That's where a small pricing or cost fix has the biggest impact, precisely because it's selling in volume.
What to do once a leak is found
Finding the leak is the useful part, fixing it is usually one of a few straightforward moves: adjust the price, renegotiate the supplier cost behind it, pull back on the discounting or ad spend propping up its volume, shift more of its sales toward the channel where it actually performs best, or in some cases, simply stop pushing it and let a more profitable product take its place in marketing and shelf space. Before making a major change, estimate how it will affect both sales volume and total contribution, and check whether it will create additional costs or affect sales of other products.
The bottom line
Revenue tells you what's selling. It doesn't tell you what's actually making money. The only way to know the difference is to look at true margin, product by product and channel by channel, and the businesses that do this regularly can uncover unexpected results including best sellers that contribute less profit than anticipated.
This is exactly the audit we run as part of a client's monthly profitability dashboard, so the leak gets caught before a whole quarter of volume has quietly eaten into the margin.
See which of your products are actually making you money.