What about orders placed before I filled in the costs?

The short answer

Those orders have no cost recorded, and nothing can put it there now. Their revenue still counts; their profit does not. The coverage line says how much of your revenue that is:

Cost is known for 74% of revenue. Profit and margin above are calculated on that part only; zł1,119,760.42 of revenue has no cost recorded.

That sentence is the difference between a report you can act on and a report that looks precise and is quietly wrong by a quarter.

What WooCommerce actually does

The cost is recorded at the moment of sale, copied from the product onto the order line. That is the right design — it is what makes an order from 2025 still say what the goods cost in 2025, even after your supplier raised prices twice.

The consequence is that there is no back-fill. Entering a cost today does not reach into orders that are already stored, and neither WooCommerce nor any plugin can reconstruct what you paid for goods before you wrote it down anywhere.

What to do about it

• Start the clock now. Enter the costs, then judge the report on the period after that point. A month of complete data beats a year of guesses.
• Pick a period that matches your data. If costs went in on 1 March, a report of March onwards is worth reading, and a twelve-month view will always carry the older gap.
• Do not write today’s prices into old orders. It is tempting and it produces a record that cannot be checked against anything. For a rough view of that period, Merchgain 0.2.5 and later shows an estimate from today’s costs in a panel of its own, apart from the recorded figures.

How Merchgain reports it

An item with no cost counts towards revenue, is left out of profit and margin, and is never treated as if the goods were free — which would inflate the margin instead of admitting the gap.

The coverage line closes the summary panel, under the revenue bar, with the uncovered amount in money, not only as a percentage. Rows that mix known and unknown costs are marked in the tables, so a suspiciously good margin can be traced to its cause in one glance.

From version 0.2.5, if the product has a cost today, the item also gets an estimate: in a panel of its own under the coverage line, marked as an estimate and never added to profit or margin.