The money layer
Why your Shopify margins are wrong
Last updated 22 July 2026. Written by Purser.
Shopify stores one number for what a product costs you. It is called cost per item, and it holds the last price you paid.
100 units at $10, then 100 more at $14
- Shopify: cost per item
- $14.00
- What you actually paid, on average
- $12.00
The cost is the last price, not your cost
You buy 100 units at $10. Later you buy 100 more at $14, because your supplier raised prices or the exchange rate moved. You now hold 200 units that cost you $2,400, an average of $12 each.
Shopify says they cost $14 each.
Every margin figure you read is now calculated against $14, on stock that mostly cost you $10. Sell the cheap units first and you will under-report your profit. Get one cheap closeout delivery and every report flatters you until the next order arrives.
What weighted-average cost does instead
Weighted-average cost keeps the average of everything you actually paid for the stock you actually hold, and updates it when stock arrives rather than when someone edits a field.
A standard weighted-average calculation: ((current average cost × current stock quantity) + (cost of received stock × received quantity)) ÷ (current stock quantity + received quantity).
30 units on hand at an average of $10, then 15 arrive at $8: ((10 × 30) + (8 × 15)) ÷ (30 + 15) = $9.33.
Purser runs this calculation on every receipt, including partial deliveries, and keeps the history of each delivery behind the average, so you can see how the number got where it is.
Why Stocky users notice this immediately
Stocky calculated a number like this and called it average unit cost. Shopify's documentation says: "Average unit costs are unique to Stocky and don't display in Shopify."
When Stocky stops working on 31 August 2026, shops that relied on it go back to a single cost per item unless something else calculates an average. That is the specific hole Purser was built to fill.
Shopify shuts Stocky down on 31 August 2026
Stocky stops working on 31 August 2026
The supplier records, costs and purchase-order history you built up over years do not move to Shopify by themselves. Export them, bring them into Purser, and keep the ledger you have been running your shop on.
Free plan, five purchase orders a month. Pro is $37.99 a month, flat.