Most COGS numbers are wrong in the same way
Ask a store owner what a product costs and you will usually get the supplier's unit price. That number is on the invoice, it is easy to find, and it is almost always too low. The shipment also carried freight, and probably duty, and possibly inspection and brokerage on top. Those charges are real money paid to acquire the same goods, so accounting standards treat them as part of cost of goods sold. Leaving them out inflates your gross margin by whatever they came to, which for imported physical products is regularly ten to thirty percent of the goods value.
The reason people leave them out is not laziness, it is that the charges arrive as one lump for a whole shipment. A single freight bill covers forty different SKUs. There is no obvious way to say how much of it belongs to the mugs versus the kettles, so the freight bill gets filed under shipping expenses and the product cost stays at the supplier price. The margin on every product then looks better than it is, and the products that are genuinely marginal stay invisible.
This calculator does the allocation for you. Enter the lines from your supplier invoice, add each charge once, and pick how it should be shared: by line value, by units, or by weight. Value is the sensible default for duty, since duty is normally assessed on declared value. Weight is usually the honest choice for sea and air freight, because that is what the carrier actually charged on. Units make sense when everything in the box is roughly the same size.
The result is a landed cost per unit for each line, and a set of totals that reconcile exactly to the cent. That is the number to put into your accounting, your pricing model, and your profit reporting. If you also enter your sell prices, you will see the gross margin each product genuinely earns, which is frequently a less comfortable number than the one you were working from.