Most small retail systems store one field called cost price against each product. It is simple, it is what a spreadsheet naturally does, and it produces wrong numbers the moment a supplier raises a price.
What actually happens
You hold forty cartons bought at ₦12,000. The supplier raises the price and you receive twenty more at ₦14,000. Your system now shows one cost: ₦14,000. Not just for the new cartons — for all sixty, and for everything you sold last month too, because the report calculates profit by looking up today's cost figure.
So last month's profit changes. Not because anything happened last month, but because a price changed this month. Your stock valuation is wrong in the same direction: sixty cartons valued at ₦840,000 when you paid ₦728,000 for them.
The fix is layers, not a better single number
Each delivery should be recorded as its own layer carrying the price actually paid on that date. When goods are sold, the sale consumes layers in an order you have chosen — first in first out, or weighted average — and writes the resulting unit cost onto the transaction itself.
Once the cost is frozen onto the sale, nothing that happens afterwards can move it. Last month's profit is final. Stock on hand is valued at what those specific goods cost. A write-off is costed at the price of the batch actually discarded rather than at today's price.
Which method to choose
First in first out matches how perishable stock physically moves and gives a stock valuation closer to current prices. Weighted average smooths volatility and is simpler to explain to staff. Either is defensible; using neither is not.
The second benefit nobody expects
Once every delivery is a batch with its own date, expiry belongs to the batch rather than to the product. Restocking a product no longer moves the expiry date of stock you have been holding for two months. Alerts become accurate, and disposal is written off at the correct cost.
That is why Shevy Kalku records deliveries as cost layers rather than overwriting a single figure. It is not an accounting nicety. It is the difference between a profit statement you can act on and one that changes behind you.