Inventory Costing Methods for E-Commerce Sellers: FIFO, Average Cost & More

Inventory costing determines how much of a product purchase remains on the balance sheet and how much moves to cost of goods sold when an item sells. For an e-commerce seller, that choice affects gross margin, ending inventory, reorder decisions, and the comparability of monthly reports. The goal is not to pick the method that produces the most flattering month; it is to use a method that fits the inventory and is applied consistently.

Tax accounting rules and financial-reporting requirements can depend on the business, entity, inventory volume, and applicable standards. Confirm the method and any required elections with a qualified tax professional. This article is general bookkeeping information, not personalized tax advice.

First, separate inventory from COGS

When merchandise is purchased for resale, the purchase is generally tracked as inventory until the related goods are sold. When the sale occurs, the cost assigned to those units moves from inventory to COGS. COGS is not the same as the supplier bill's payment date. Freight-in, tariffs, landed costs, packaging, and other costs may need to be considered in the inventory-cost policy, while outbound shipping and marketing often follow different categories. The important point is to define the policy and apply it consistently.

A simple example: a seller buys 100 units at $10 each and sells 30. Under a basic perpetual workflow, $300 moves to COGS and $700 remains in inventory, before considering additional included costs. If the seller expenses the entire $1,000 purchase immediately, that month's profit is understated and the balance sheet no longer shows the goods still available for sale.

FIFO: first costs assigned first

FIFO, or first-in, first-out, assigns the oldest available inventory costs to units sold first. It is often intuitive for physical goods that move in chronological order, especially products with expiration, style changes, or a real risk of older stock becoming obsolete. The accounting convention does not require the warehouse picker to grab the oldest box every time; it is a costing rule that assigns costs in an orderly way.

Suppose a SKU has 40 units at $8 and then 40 units at $10. If 50 units sell under FIFO, COGS is $40 × $8 plus $10 × $10, or $420. The remaining inventory is 30 units at $10, or $300. When supplier prices rise, FIFO often leaves newer, higher costs in ending inventory and assigns older, lower costs to current sales. That can make current gross margin look higher than it would under a method using more recent costs.

Weighted average cost: smooth the layers

Weighted average cost combines the cost of available units and divides by the available quantity to produce an average unit cost. In the same example, 80 units cost $720 total, so the average cost is $9 per unit. Selling 50 units assigns $450 to COGS and leaves 30 units at $270. This smooths price changes and can be practical for sellers with many similar units and frequent replenishment.

Average cost is easier to maintain when individual purchase layers do not matter operationally, but it still needs accurate quantities, receipts, returns, and landed-cost treatment. A spreadsheet that updates the average only when a bill is paid can be wrong if goods were received earlier or if a supplier credit changes the cost basis.

Specific identification: match the actual unit

Specific identification assigns the actual cost to the particular item sold. It is most useful when each item is unique, high-value, serialized, customized, or otherwise tracked individually. A seller of one-of-a-kind furniture, for example, can identify the purchase cost of the exact piece shipped. It is usually less practical for hundreds of interchangeable phone cases or common-sized apparel units unless the inventory system tracks serial or lot information.

Specific identification can provide excellent product-level visibility, but it requires reliable item records. If the SKU, lot, serial number, and purchase cost are not connected to the sale, the method becomes a manual guess rather than an auditable process.

What about LIFO and “retail” methods?

LIFO, or last-in, first-out, assigns newer costs to sales first. Whether it is permitted or appropriate depends on the reporting framework and tax rules that apply to the business; it should not be adopted casually because it changes the treatment of inventory layers. Likewise, a retail inventory method can be useful for certain larger retail operations that track inventory at selling price and maintain a cost-to-retail relationship, but it requires a structured policy and reliable retail data. Small sellers should discuss these methods with their tax professional before using them.

Consistency matters more than chasing a monthly result

Changing from FIFO to average cost, or changing which freight and packaging costs are included, can make two months look more different than the underlying business actually is. A consistent policy makes gross margin trends meaningful. If a change is necessary, document the reason, effective date, impact on inventory and COGS, and who approved the change. Do not quietly change methods because one approach produces a preferred profit number.

Build a practical e-commerce costing workflow

1. Maintain a SKU master with product name, SKU, units, category, and any lot or serial fields needed.
2. Record quantities received and vendor costs, including the treatment of freight-in or other landed costs in your policy.
3. Connect orders, refunds, replacements, bundles, and returns to inventory quantities.
4. Choose FIFO, weighted average, specific identification, or another appropriate method with professional input.
5. Reconcile the inventory subledger to the general ledger at month-end.
6. Review negative quantities, unexplained adjustments, damaged goods, shrinkage, and stale stock separately.

For a Shopify seller also using Amazon or Etsy, the costing method should not change merely because the order came through a different channel. The channel is useful reporting detail; the same SKU and cost policy should drive COGS across channels unless there is a documented reason otherwise. That is how channel gross-margin comparisons stay useful.

Example: why channel margin can be misleading

A seller buys a product in two batches: 100 units at $12 and 100 units at $16. Shopify sells 80 units, while Amazon sells 80 later. If the seller records all purchases as expense when paid, neither channel has a meaningful COGS result. If the seller uses a defined costing method and records sales-related COGS consistently, the reports can show product margin by channel while keeping unsold inventory on the balance sheet. Shipping, marketplace fees, and advertising can then be analyzed separately rather than hidden inside product cost.

Linked-do-Books helps Shopify-led and multi-channel sellers organize inventory activity, COGS, payout reports, fees, and month-end reconciliations into a clearer financial picture. See how processor payout reconciliation fits with revenue reporting and how to organize sales-tax nexus records across channels.