Can the report show profit for orders placed before I entered my costs?

The short answer

Yes, as an estimate, and only as one. An item sold before a cost was recorded on the order is priced at what its product costs today, times the quantity sold. That is the figure WooCommerce itself would record if the same sale happened now.

The estimate has a panel of its own under the summary cards and two columns of its own in the tables. Every estimated figure in it is marked with ≈, and none of it is added to the recorded profit: the cards above it show exactly what they showed before.

What WooCommerce actually does

WooCommerce records the cost on the order line at the moment of sale. An order placed before you switched the feature on, or before its product had a cost, has no cost on its lines, and nothing writes one there later.

What the shop does have is each product’s cost today. The estimate reads it by the same rule WooCommerce applies when it records a cost:

• a variation with a cost of its own uses it; without one it takes its parent’s; with the “additive” setting the parent’s cost and the variation’s are added up;
• an item whose variation has since been deleted falls back to the parent product, because that is the product WooCommerce reads for it;
• a cost of zero, a product with no cost and a product deleted since the sale give no estimate, because WooCommerce would record nothing for them either;
• a refund of such an item is estimated the same way, as a negative amount in the period the refund was issued.

When the estimate misleads

When your cost price changed since the sale. The estimate is off by exactly that change. If a supplier raised prices by a tenth in spring, last winter’s orders get a cost a tenth too high and a margin that is too low. The older the order, the more room there is for that.

When today’s cost is a guess. A cost entered as a round number, or copied from a similar product, goes straight into every estimated line of that product.

So read it for which products earned and roughly how much. It is not a record of what you paid, and for the books only the recorded figures count.

How Merchgain reports it

From version 0.2.5, the report shows a panel marked Estimate under the coverage line: the revenue it covers, the estimated cost of goods, gross profit and margin, and how many items it is built from. If part of the revenue cannot be estimated either, the panel says how much.

The product and category tables get two more columns, Estimated profit and Estimated margin, and both can be sorted. A row with nothing to estimate shows a dash, not a zero. The CSV export carries five columns ending in _estimated and a line at the end saying what they mean. Profit, margin and coverage are calculated exactly as before, and costs kept by other plugins are not used here either.

Nothing is written to your shop. The estimate is worked out when the report opens, from the costs on your products, so it follows the moment you change a cost.