E-Commerce Seller Estimated Taxes: The Fourth Quarter That Is Half the Year, Inventory You Paid For but Haven't Sold, and the Payout Lag

E-commerce sellers meet the estimated tax system with a year that happens in November and December and a cash flow that runs backward from the profit. The rules (the contractor estimated-tax guide covers the mechanics): quarterly installments on April 15, June 15, September 15, and January 15; a quarter-by-quarter underpayment penalty; avoided by the prior-year safe harbor (100% of last year's tax, 110% above US$150,000 of prior-year adjusted gross income) in equal installments, 90% of the current year's tax in equal installments, or the annualized method. The income shape: sales build through the year with a summer plateau (or a summer dip for some categories), a run-up from the marketplace's mid-year sales event, and a fourth quarter — the holiday season from the marketplace's fall promotions through December — that can be 40 to 50 percent of annual sales for seasonal and gift-driven consumer categories (lower for consumables and business products), followed by a January of returns; so the year's profit is concentrated in the fourth quarter, the first three quarters' installments under equal payments overpay relative to income earned, and the annualized method (installments on year-to-date profit through each cutoff — small in April and June, moderate in September, large in January) fits the shape exactly for a seller whose books are current at each quarter's cutoff. The two cash distortions. Inventory bought before it sells: the holiday inventory is ordered in the summer and paid for in August through October (the manufacturer's deposit and balance, the freight, the tariffs — the e-commerce deductions guide's landed cost) — cash out of six figures for a mid-sized seller, with no deduction until the units sell in November and December (inventory is deductible as cost of goods sold when sold, not when bought), so the third quarter's cash is deeply negative while its taxable profit is unchanged, and a seller who projects on cash (purchases as expenses) under-projects the year's tax and over-deducts the third quarter; the projection runs on cost of goods sold (units sold times landed cost), computed from the sales reports and the inventory ledger, not on the purchase invoices. The payout lag: the marketplace settles every two weeks (or on its own schedule), and the seller's own site's processor settles in days — so December's sales are largely January's cash — but not January's income: the marketplace and the processor collect the buyer's payment as the seller's agent, and under the cash method income is received when the seller's agent receives it, so December's sales are December's income under either method; the lag is a cash-flow problem (the fourth installment or the December withholding is funded before the December payouts arrive), not a timing shift between years. The holiday buy and the fall recompute: the size of the holiday inventory buy is decided in the summer, and its cost of goods sold lands in the fourth quarter — the fall recompute (October) rebuilds the year's projection from the actual holiday inventory position, the fourth-quarter sales forecast, the advertising budget for the season (a large fourth-quarter cost), and the fee schedule, and sets the January installment (the year's largest under the annualized method) or the S corporation owner's December withholding. The two strategies. The annualized method: installments on year-to-date profit through March 31, May 31, August 31, and December 31, annualized — the natural fit for a back-loaded year; requires the books current at each cutoff (the sales reports, the fee reconciliation, the cost of goods sold from the inventory ledger — the accounting integration that pulls marketplace data into the books makes this routine); Form 2210 Schedule AI at filing. The prior-year safe harbor with a reserve: four equal installments of last year's tax — penalty-proof, with a reserve percentage of every payout (for a seller with a 12 percent net margin on gross and a 30 percent effective rate, about 3.6 percent of gross sales — roughly 5 percent of a marketplace payout that arrives net of its fees) funding the installments and the April balance in a growth year; the choice for a seller whose year looks like last year and whose books aren't current enough to annualize — with the caution that a growth year's April balance is large and the reserve has to have grown with the fourth quarter's payouts, which arrive in January. The S corporation seller (the e-commerce entity guide): the owner's salary withholding covers the tax on salary and projected distributions — deemed paid evenly across the year regardless of when withheld — with the fall recompute setting the December payroll's withholding for the fourth quarter's profit; the mechanism that turns the back-loaded year into a December W-4 adjustment; a seller with a warehouse team runs the payroll anyway. What the estimate includes: federal income tax on projected profit (from cost of goods sold, not purchases); self-employment tax for a Schedule C seller (the omitted third); the state's estimates (and the multistate income tax where the fulfillment map creates it — the e-commerce entity guide); the QBI deduction (not a specified service trade); the fourth quarter's advertising (a cost that scales with the season); the returns in January (a reduction of next year's sales, with the returned units back in inventory); the fee schedule's seasonal surcharges (the marketplace's storage fees rise in the fourth quarter); and any equipment or software capitalized. The quarterly check: sales by channel against the seasonal projection; the inventory position (units on hand and in transit, and the holiday buy's status); cost of goods sold computed; advertising spend; the fee reconciliation; the payout lag's cash effect at year-end; profit through the cutoff annualized against installments or withholding; the reserve balance; and the adjustment. The failure modes: projecting on purchases (the third quarter's holiday buy treated as a deduction — the year under-projected and the January installment a shock); equal installments through a back-loaded year (overpaying April and June, with the cash needed for the holiday buy); the December sales pushed into next year because the payouts arrived in January (the marketplace received the buyers' payments as the seller's agent in December); the fourth quarter's advertising left out of the projection; omitting self-employment tax; and the January returns not in the following year's first-quarter computation. The calendar: January 15 — the fourth installment (the year's largest under the annualized method); late January — last year closed (the year-end count, December's sales reconciled to January's payouts, the 1099-Ks reconciled), the safe harbor computed, the reserve percentage set (or the S corporation W-4), the seasonal projection built from the sales history; each payout — reserve by rule; quarterly — the check at each cutoff (March 31, May 31, August 31 for Schedule AI); April 15, June 15, September 15 — installments (small, small, moderate under annualized); summer — the holiday buy sized and its cash planned; October — the fall recompute (the holiday inventory, the fourth-quarter forecast, the advertising budget, the fee surcharges); December — the withholding cure (S corporation) and the count; filing — Form 2210 Schedule AI.

Key takeaways

• The fourth quarter can be 40–50 percent of the year for seasonal and gift-driven categories — the annualized method fits a back-loaded year exactly; equal installments overpay the first half when the cash is needed for the holiday buy.
• Project on cost of goods sold, not purchases: the holiday inventory paid for in August–October is cash out with no deduction until it sells in November–December.
• The payout lag is a cash problem, not a timing shift: the marketplace collects as the seller's agent, so December's sales are December's income even when the payout lands in January.
• The fall recompute rebuilds the year from the actual holiday inventory position, the fourth-quarter forecast, and the season's advertising — setting the January installment or the December withholding.
• S corporation sellers turn the back-loaded year into a December W-4 adjustment; Schedule C sellers include self-employment tax and reserve a margin-calibrated share of every payout.
• January's returns reduce next year's first-quarter sales; the fourth quarter's storage surcharges and advertising belong in the projection.

The e-commerce seller's estimated-tax calendar

January 15: fourth installment (the year's largest). Late January: last year closed (count, December sales to January payouts, 1099-Ks); safe harbor; reserve or W-4; seasonal projection from history. Each payout: reserve by rule. Quarterly cutoffs (Mar 31, May 31, Aug 31): sales by channel; inventory position; COGS computed; advertising; fees; profit annualized; adjust. April 15, June 15, September 15: installments. Summer: holiday buy sized. October: fall recompute — holiday inventory, Q4 forecast, ad budget, surcharges. December: withholding cure; count. Filing: Schedule AI. The COGS-not-purchases line is the one that decides whether the projection is right.

Worked example

A consumer-products seller (S corporation, three employees) projects US$240,000 of profit on US$1.9 million of gross: 12 percent of sales in Q1, 18 percent in Q2, 22 percent in Q3, 48 percent in Q4. Last year's tax was US$58,000; the owner's salary withholding is set low in January (matching the thin first half), and 3.6 percent of gross sales is reserved from every payout by rule. August–October: the holiday inventory buy — US$310,000 of product, freight, and tariffs paid to the manufacturer and the forwarder — is cash out that the projection ignores as a deduction (the units go into the inventory ledger at landed cost; cost of goods sold follows the sales); the bank balance falls to its low point in October while the year-to-date taxable profit through September 30 (about US$95,000 on 52 percent of the year's sales) is unchanged by the buy. The October recompute: the holiday inventory position, a fourth-quarter forecast of US$910,000 of sales, US$74,000 of fourth-quarter advertising, and the marketplace's Q4 storage surcharges — the year's profit rebuilt at US$255,000, and the December payroll's withholding set to cure the year (deemed paid evenly, so the low first-half withholding is not an underpayment). December 31: US$140,000 of December sales whose payouts settle in January stay in this year's income (the marketplace collected them as the seller's agent), so the December withholding is funded before that cash arrives, and the year-end count (US$180,000 of closing inventory across the warehouse and four fulfillment centers) sets cost of goods sold. January: US$38,000 of returns reduce next year's first-quarter sales, with the restockable units back in inventory. The seller across the marketplace, on a Schedule C with equal installments and a projection built on purchases: overpaid April and June while borrowing for the holiday buy, "deducted" US$300,000 of inventory in the third quarter, and met a January installment three times the size of the one he'd computed.

Official sources

The IRS states: “Individuals, including sole proprietors, partners, and S corporation shareholders, generally have to make estimated tax payments if they expect to owe tax of $1,000 or more when their return is filed.” — Internal Revenue Service, Estimated taxes, https://www.irs.gov/businesses/small-businesses-self-employed/estimated-taxes

Publication 538 states: “An inventory is necessary to clearly show income when the production, purchase, or sale of merchandise is an income-producing factor. If you must account for an inventory in your business, you must use an accrual method of accounting for your purchases and sales.” — Internal Revenue Service, Publication 538, Accounting Periods and Methods, https://www.irs.gov/publications/p538

Practitioner note

An e-commerce seller's estimated taxes run on a year that happens in November and December and a cash flow that runs backward from the profit: the holiday inventory paid for in August is cash out with no deduction until it sells, and December's sales are January's payouts. Our e-commerce routine projects on cost of goods sold rather than purchases, annualizes a back-loaded year so the first half's installments stay small while the cash funds the holiday buy, and rebuilds the year in October from the actual inventory position and the fourth-quarter forecast — because the seller who deducted the third quarter's purchases met a January installment three times the size he'd computed.

See also: For related guidance, see the qualified business income deduction, explained; and browse every small business tax guide, by situation.

Next step

Fairlight Accounting is a cross-border accounting and tax practice with a U.S. Tax Desk and a Canadian Tax Desk. Our U.S. Tax Desk handles estimated-tax planning for e-commerce sellers — annualized-method setups for back-loaded years, cost-of-goods-sold projections with inventory ledgers, payout-lag cash planning at year-end, holiday-buy cash planning, fall recomputes with Q4 advertising and fee surcharges, and S corporation withholding cures. See pricing or book a call.