1. What Is COGS?
COGS — Cost of Goods Sold — is the direct cost of acquiring the inventory you have already sold. It is not what you spent on inventory this month. It is the cost that gets matched to revenue when a unit leaves your hands and reaches a customer.
This distinction matters. If you buy 200 units in January but only sell 80 of them, your COGS for January is the cost of those 80 units — not 200. The other 120 units remain on your balance sheet as an asset (inventory) until they sell.
Amazon Seller Central does not have a COGS field. It shows you revenue and Amazon-side fees, but has no concept of what you paid for the inventory. That gap is where most seller P&L reports break down.
2. What Counts as COGS (and What Doesn't)
Getting this boundary right is the foundation of accurate margin reporting. Misclassifying operating expenses as COGS — or vice versa — distorts your gross margin and makes it impossible to benchmark your sourcing performance.
Included in COGS
| Cost Item | Why It's COGS |
|---|---|
| Product purchase price | The supplier or store invoice amount per unit |
| Inbound freight to Amazon | Direct cost to get goods into FBA; part of the cost basis |
| Import duties & customs fees | Required to bring the product to a sellable state |
| Prep center costs | Labeling, poly-bagging, bundling — transforms the item for FBA |
| Inspection & QC fees | Direct cost tied to specific inventory lots |
| Supplier-side shipping charges | FOB, CIF, or freight charges billed on the purchase order |
Not COGS — These Are Operating Expenses
| Cost Item | Correct Classification |
|---|---|
| FBA fulfillment fees | Operating expense — cost of selling, not acquiring |
| Amazon referral fees | Operating expense — Amazon's commission per sale |
| Monthly FBA storage fees | Operating expense — holding cost, not unit cost |
| Amazon advertising (PPC) | Operating expense — marketing spend |
| Subscription fee ($39.99/mo) | Operating expense — fixed overhead |
| Returns processing fee | Operating expense — post-sale cost |
| Software tools & subscriptions | Operating expense — general overhead |
3. Where Amazon Sellers Get COGS Wrong
The most common COGS mistakes aren't careless — they're the natural result of how Amazon sellers operate: multiple suppliers, changing prices, mixed fulfillment channels, and no built-in cost field in Seller Central.
4. Inventory Costing Methods Compared
There are three main methods for assigning a cost to each unit sold. Each produces a different COGS number from the same set of purchases.
| Method | How It Works | Best For | Main Risk |
|---|---|---|---|
| FIFO First In, First Out | Cost of oldest lot is assigned to each unit sold first | Most Amazon sellers; lot-based buying; changing prices | Requires tracking each purchase lot separately |
| Weighted Average | Blends all purchase costs into one per-unit rate | Stable-price products; simple operations | Masks price volatility; misrepresents per-lot margin |
| LIFO Last In, First Out | Cost of newest lot is assigned first | Not used in e-commerce; banned under IFRS | Not GAAP-compliant outside US; rarely appropriate for Amazon |
| Specific ID | Each individual unit is tracked to its exact purchase cost | Very high-value or serialized items | Impractical at scale for commodity products |
FIFO is the standard for Amazon sellers — and the most defensible for tax purposes. It most closely mirrors how physical inventory actually moves: older stock tends to sell (or sit) before newer stock, especially in FBA where Amazon controls fulfillment order.
5. FIFO Step by Step: A Real Example
Let's walk through FIFO with a concrete scenario so the mechanics are clear.
Scenario: Wholesale seller, one ASIN, two purchase lots
| Date | Event | Units | Unit Cost | Total Cost |
|---|---|---|---|---|
| Feb 1 | Purchase – Lot A | 100 | $5.00 | $500.00 |
| Feb 15 | Purchase – Lot B | 100 | $6.50 | $650.00 |
| Feb 1–28 | Sales | 120 | — | — |
Applying FIFO to the 120 units sold:
What happens when you add a third lot?
Suppose in March you buy Lot C: 150 units at $5.80. You sell 90 more units in March. FIFO applies the 80 remaining Lot B units first, then takes 10 from Lot C:
6. FIFO vs. Weighted Average Cost: The Difference in Numbers
Using the same two-lot scenario (100 @ $5.00, then 100 @ $6.50, selling 120 units), here's how the two methods diverge:
FIFO
Units sold: 120
COGS: $630.00 ($5.25/unit)
Ending inventory: 80 units @ $6.50
Inventory value: $520.00
Weighted Average
Avg unit cost: $5.75 ($1,150 ÷ 200)
COGS: $690.00 (120 × $5.75)
Ending inventory: 80 units @ $5.75
Inventory value: $460.00
| Metric | FIFO | Weighted Avg | Difference |
|---|---|---|---|
| COGS (120 units) | $630.00 | $690.00 | −$60.00 |
| Gross Profit (at $12 sell price, 120 units = $1,440 revenue) | $810.00 | $750.00 | +$60.00 |
| Gross Margin % | 56.3% | 52.1% | +4.2 pts |
| Ending Inventory Value | $520.00 | $460.00 | +$60.00 |
The total profit over the full lifecycle is the same — the $60 difference is timing. Under FIFO, that $60 flows through when the Lot B units sell. Under average cost, it's blended in now. For a single two-lot scenario this looks minor. Across 50 ASINs with monthly restocks, the distortion compounds.
7. Real-World P&L Impact
Here's what a full P&L looks like with proper COGS treatment vs. a rough estimate. Assume a seller moves 120 units at $12.00 (revenue: $1,440), with Amazon fees totaling $5.40/unit ($648 total).
| Line Item | With FIFO COGS | With Estimated COGS |
|---|---|---|
| Revenue | $1,440.00 | $1,440.00 |
| COGS | $630.00 | $720.00 (guessed $6/unit) |
| Gross Profit | $810.00 | $720.00 |
| FBA Fulfillment Fees | −$360.00 | −$360.00 |
| Referral Fees (15%) | −$216.00 | −$216.00 |
| Storage + Other Fees | −$72.00 | −$72.00 |
| Net Profit | $162.00 | $72.00 |
| Net Margin | 11.3% | 5.0% |
A $90 COGS error on 120 units produces a reported net margin that's less than half the real number. Scale that across thousands of units and you're making sourcing decisions — and tax estimates — on numbers that don't reflect reality.
8. Tax Implications of COGS and FIFO
COGS directly reduces your taxable income — but only in the period the goods actually sell. This is accrual-basis accounting: the cost is matched to the revenue it generates.
- Inventory is an asset, not an expense. When you buy 200 units for $1,000, you don't deduct $1,000 on your taxes immediately. You deduct it as the units sell (as COGS).
- FIFO affects which period's tax bill you reduce. If your costs are rising, FIFO assigns cheaper older costs to this period's COGS, leaving a higher-cost inventory asset that will reduce a future period's taxes. This is legal and expected.
- Unsold inventory at year-end has real tax consequences. If you have $30,000 of inventory that didn't sell by December 31, you don't get to deduct that cost this year — it remains an asset and reduces next year's taxes when it sells.
- Consistent method matters. The IRS requires you to use the same costing method year over year. Switching from average cost to FIFO requires a change in accounting method (Form 3115). Use FIFO from the start if that's your intent.
9. How to Track COGS Accurately
Manual FIFO tracking in a spreadsheet is possible for small catalogs, but it breaks down fast as you add ASINs, multiple lots, and return adjustments. Here's what proper COGS tracking looks like:
What you need to capture per purchase lot
| Field | Purpose |
|---|---|
| ASIN / SKU | Link cost to the right product |
| Purchase date | Establishes lot age for FIFO ordering |
| Units purchased | How many units are in this lot |
| Purchase price (per unit) | The base cost |
| Inbound freight (per unit) | Allocate shipping cost across units in the shipment |
| Duties & prep (per unit) | Any additional costs to get the unit to FBA |
| Total cost basis (per unit) | Sum of all the above — this is your FIFO unit cost |
| Units remaining in lot | Updated as units sell; tells you when to move to the next lot |
Allocating freight across units
The cleanest method is to divide the total freight cost by the total units in the shipment. If you ship 300 units and pay $180 in freight, that's $0.60/unit added to each unit's cost basis — regardless of which ASIN or lot they belong to.
Handling returns
When Amazon returns a unit to your sellable inventory, it should be added back to the lot it came from (or to a new lot at its original cost basis). If the unit is unfulfillable and you have it sent back to you, it remains an asset at cost until you dispose of it — at which point the loss flows through as an expense.
FAQ
Does Amazon Seller Central track COGS for me?
Can I use average cost instead of FIFO?
Should inbound shipping to Amazon be included in COGS?
What happens to COGS when Amazon loses or damages my inventory?
Is FIFO required, or just recommended?
Know your real COGS. Know your real profit.
SellerGuards tracks inventory by purchase lot and applies FIFO automatically — so your P&L reflects what you actually earned, not an approximation.
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