Cost of goods sold is the biggest number standing between an ecommerce seller's revenue and their taxable profit, and yes, it reduces your taxes dollar for dollar. Technically the IRS treats COGS as a reduction of gross receipts rather than a business expense like rent or ads, but the effect on your bill is the same and often larger: a store with $500,000 in sales and $200,000 of COGS is taxed on the margin, not the top line.
A subtraction computed in Schedule C Part III
Sole proprietors and single-member LLCs calculate COGS in Part III of Schedule C using a formula: beginning inventory, plus purchases and other product costs during the year, minus ending inventory. Whatever that formula produces flows to the front of Schedule C and comes off gross receipts before your other expenses are even considered. Partnerships and corporations run the same math on Form 1125-A attached to their returns.
The formula is also the answer to the timing question sellers ask most. Buying $50,000 of stock raises purchases, but if none of it sold, it also raises ending inventory by $50,000, and the two cancel. The deduction only materializes as units leave inventory through sales. Restocking in December does not shrink December's tax bill; the write-off arrives across next year as the units sell. This holds even for cash-method sellers: under the IRS small-business rules, inventory treated as non-incidental materials and supplies is still deducted when used, meaning when sold, not when paid for.
What belongs in COGS for an online store
For a product brand, COGS includes more than the factory invoice. Count the per-unit product cost, inbound freight and duties to get goods to your warehouse or 3PL, packaging that ships with the product, and manufacturing labor or materials if you produce goods yourself. Consistent practice matters more than perfection at the margins: pick a costing approach, document it, and apply it every year.
Some costs sit outside COGS and are deducted as ordinary expenses instead, which is actually better for timing since they are not tied to when units sell: outbound shipping to customers, marketplace referral fees, payment processing, ad spend, and software. Misfiling those into COGS delays deductions you could have taken immediately.
Two housekeeping rules make the number defensible. First, count ending inventory for real at year end, reconciling Shopify or FBA valuations against a physical count; the ending inventory figure directly sets the size of the deduction. Second, clear dead stock deliberately: damaged, expired, or unsellable goods removed from inventory increase COGS in the year you dispose of them, turning shelf failures into at least a tax benefit. A seller who never writes off dead stock is quietly overstating profit and overpaying tax every year the units sit there.