Keeping your average cost right when a delivery arrives

The arithmetic, in one line

Weighted average cost = (units on hand × current cost + units received × price paid) ÷ total units.

Concretely: 12 pairs of socks on the shelf at 7.20, a delivery of 6 at 9.00, gives (12 × 7.20 + 6 × 9.00) ÷ 18 = 7.80. That 7.80 is what should be sitting against the product afterwards.

Why it matters more than it looks

• Your profit reports use the cost field. A stale cost means a margin that reads better than reality.
• Reorder decisions get made on the wrong number, so you keep buying the line that stopped being profitable.
• At stocktake, your inventory valuation is wrong — which matters to your accountant, not just to you.

Doing it without a spreadsheet

RestockDesk asks one extra question when you receive — what did you actually pay — then computes the weighted average and writes it back to the product in Shopify. Nothing else about the product is touched.

If the price did not change, the cost does not move, and you never see a prompt about it.

Questions

Does this change my retail prices?

No. It only updates the cost stored against the inventory item, which feeds your margin reporting. Your selling prices are untouched.

What if I receive the same delivery twice by mistake?

Shopify now requires the quantity a change is made from, so a duplicate submission is rejected rather than silently doubling your stock.