Weighted Average vs Last Cost: Choosing a Costing Method for Your Shopify Store

The Three Choices, Plainly

Smart Restock's costing setting is shop-wide, because Shopify keeps one cost per inventory item and the write is global. It has three values:

One rule applies to all three: a $0 line, such as a free sample or a 100%-discounted item, never writes a cost. It would otherwise drag your average toward zero or wipe out the cost entirely. For a deeper look at the blending maths, see our guide to weighted average cost in Shopify .

Same Two Invoices, Three Different Costs

Start with the drill bit set: 40 on hand at $10.00. Invoice one brings 60 units at $12.00. Invoice two, a couple of weeks later, brings 50 units at $11.00. Here's the cost field after each delivery, with the retail price held at $20.00:

Look at the middle column. After invoice one, last cost says your margin is 40% and weighted average says 44%. Both are arithmetically right, and they answer different questions. Last cost tells you what a unit costs to replace today. Weighted average tells you what the units on your shelf actually cost you. And the third row is a warning: leave cost alone and your reports show a 50% margin on an item that's now being bought at $11 to $12.

When Weighted Average Is the Right Call

Most merchants should stay on the default, and the reason is boring in a good way. Weighted average smooths out supplier noise, so a single odd delivery doesn't whipsaw your margin report. It also tends to be the method an accountant will recognize for a store that holds general stock: the cost of what's sitting in the back room is the blend of what you paid for it.

It fits best when you reorder the same SKUs repeatedly, when stock turns over steadily, and when you sell from a single pool of identical units. Hardware, pet supply, grocery and general auto parts all look like this. If a bolt bought in March is physically interchangeable with a bolt bought in June, an average is a fair description of what's in the bin.

The trade-off is lag. Because new cost gets diluted by old stock, the stored cost moves more slowly than the supplier's price. We covered why that matters for spotting increases in tracking supplier price creep : the invoice line is the early signal, and the blended number is the one that drives your books.

When Last Cost Makes More Sense

Last cost is the replacement-cost view. It suits stores whose supplier prices move in step changes and whose pricing follows the supplier's latest number, such as an apparel wholesaler working from seasonal price lists, or a liquor store where the distributor updates the whole book monthly and shelf prices follow. If you reprice to whatever the newest invoice says, a blended cost can make your reported margin look healthier than your next order will actually deliver.

It's also the simplest to explain and audit. The cost on the product is the cost on the most recent invoice you posted, full stop. The downside mirrors weighted average's strength: it's volatile. One delivery with a temporary promo price, or a one-off surcharge baked into a line, becomes your cost until the next invoice replaces it. And it understates the cost of older, more expensive stock you're still selling through.

When to Choose "Don't Update Cost"

This option is for merchants who manage cost somewhere else: a spreadsheet, an accounting system, or standard costs set once a quarter. Smart Restock then does what you most need from it, which is parsing the invoice, matching lines and checking the stock in, without writing to a field you own.

There's a knock-on effect worth knowing about. Automatic price adjustment works off cost changes, so with costing switched off there is no cost change to react to, and margin-holding price updates have nothing to act on. The settings page says so in plain words when you choose it. Pick this option on purpose, not as a way to avoid deciding.

What's Not on the List: FIFO and LIFO

If your accountant asks for first-in-first-out or last-in-first-out costing, Smart Restock doesn't offer either one. The three choices above are the full set. FIFO and LIFO require tracking the cost of each layer of stock separately and relieving it in a specific order as units sell, which is a different mechanism from keeping one cost on a product. Stores that need layered inventory valuation for tax or statutory reporting should keep that valuation in their accounting system and use Smart Restock for what it does well: getting accurate quantities and a sensible working cost into Shopify. This isn't tax advice, so confirm the method with your accountant before you change anything.

A Quick Way to Choose

Two housekeeping notes. The setting is for the whole store, not per product or per supplier. And when a delivery arrives as a case, the cost has to be divided down to a single unit before any method can blend it, which is why case-pack cost allocation matters more than the method you pick. If you post an invoice and the resulting cost looks wrong, you can revert it and try again rather than patching quantities by hand.

See the Result Before You Post It

Each line on a parsed invoice in Smart Restock shows a projected new cost, with a short explanation under it, such as the weighted average of what's on hand plus what's received, or that the invoice cost replaces the previous one. You see what the number will be, and why, before anything is written to Shopify. That makes it easy to try a method on a real delivery, look at the lines that move the most, and decide whether the number matches how you think about your stock.

See the new cost before it posts

Run a real supplier invoice through Smart Restock and check the projected cost on each line under the method you choose, before anything is written to Shopify.

Pick the costing method that fits

Know exactly how your cost gets calculated

Choose a costing method once and every invoice you post follows it.