TITLE: September 2026 US Tax Deadlines for International Sellers: What You Need to File
If you sell to US customers from London, Manchester, Birmingham, the EU or elsewhere, September brings two important IRS deadlines.
Tuesday 15 September 2026 is the deadline for third-quarter estimated tax payments and several extended information returns.
Wednesday 30 September 2026 is the final day to submit certain Qualified Intermediary, Withholding Foreign Partnership or Withholding Foreign Trust applications for an agreement to be effective in 2026.
The IRS has also urged eligible foreign filers to start preparing now for the 2027 filing season. The transition from FIRE to IRIS may require several weeks of registration work.
This guide explains what you need to do, which forms may apply and how to organise your records before each deadline.
September deadlines at a glance
| Date | Action |
|---|---|
| 15 September 2026 | Pay the third instalment of 2026 estimated tax for eligible individuals using Form 1040-ES |
| 15 September 2026 | Pay the third instalment of 2026 estimated tax for calendar-year corporations |
| 15 September 2026 | File extended calendar-year Forms 1120-S, 1065 and 1042 where a valid six-month extension was requested |
| 15 September 2026 | File Forms 8804 and 8805 for partnerships where required |
| 15 September 2026 | Submit Form 8813 and pay any partnership tax due where required |
| 30 September 2026 | Submit QI, WP or WT applications through QAAMS for an agreement effective in 2026 |
| Before the 2027 filing season | Complete Foreign Filer TCC registration and IDES preparation where applicable |
Always check the latest IRS instructions for the form and taxpayer type involved. The IRS third-quarter tax calendar confirms the September dates.
Pay your third estimated tax instalment by 15 September
Individuals generally need to make estimated tax payments if they expect to owe at least $1,000 when they file their tax return, after considering withholding and refundable credits.
This can affect overseas business owners who receive income from a US LLC, US trade or business, partnership interest or other US-connected activity. It may also affect individuals operating ecommerce or digital businesses through a US structure.
Calendar-year corporations generally need to pay estimated tax if they expect to owe at least $500 for the year. The 15 September payment is the third instalment of the 2026 corporate estimated tax.
Use the appropriate IRS calculation method and payment process for your taxpayer type. Do not simply copy last quarter’s amount without reviewing current profit, deductions, inventory and withholding records.
Complete this payment checklist
Before 15 September:
- Update your bookkeeping through at least 31 August.
- Reconcile Amazon, Shopify, eBay, payment processors and bank accounts.
- Separate US sales from non-US sales.
- Review refunds, chargebacks, marketplace fees and fulfilment costs.
- Check inventory purchases and warehouse-related expenses.
- Review withholding already paid on your behalf.
- Recalculate your expected 2026 income and tax liability.
- Confirm the correct taxpayer identification number and payment period.
- Schedule the payment early and retain the confirmation.
If you use EFTPS, the payment must be scheduled by 8 p.m. Eastern Time at least one calendar day before the due date. For the 15 September deadline, do not wait until the evening of the deadline. Schedule the payment by the required cut-off on 14 September.
A reliable USA accounting and compliance process helps you keep the underlying records ready instead of estimating from incomplete marketplace reports.
Do not confuse estimated tax with extended returns
The 15 September IRS calendar also includes several extended filing obligations. These are separate from the third estimated tax instalment.
Calendar-year S corporations
A calendar-year S corporation must file its 2025 Form 1120-S by 15 September 2026 if it timely requested a six-month extension using Form 7004.
The extension gives additional filing time. It does not automatically extend the time to pay tax.
Calendar-year partnerships
A calendar-year partnership must file its 2025 Form 1065 by 15 September 2026 if it timely requested a six-month extension.
Where applicable, the partnership may also need to file:
- Form 8804, Annual Return for Partnership Withholding Tax.
- Forms 8805, Foreign Partner’s Information Statement of Section 1446 Withholding Tax.
- Form 8813, Partnership Withholding Tax Payment Voucher.
Review these requirements carefully if your partnership has foreign partners or income subject to section 1446 withholding.
Withholding agents
A withholding agent must file a calendar-year 2025 Form 1042 by 15 September 2026 if it timely requested a six-month extension.
Form 1042 relates to certain US-source income paid to foreign persons and associated withholding obligations. Form 1042-S records may also be relevant to the reporting process.
This is not an obligation that applies to every overseas online seller. However, it can become important where your business acts as a withholding agent, manages payments to foreign persons or operates through a structure involving US-source income.
Example: prepare your records before 15 September
Imagine a UK-based Amazon FBA business with a US LLC.
During 2026, the business has:
- US marketplace sales.
- Inventory imported into the United States.
- FBA storage and fulfilment fees.
- Refunds and chargebacks.
- Payments received from Amazon.
- A foreign owner receiving income from the business.
Before calculating the third estimated payment, the business should reconcile Amazon settlement reports to the US LLC bank account. It should also identify inventory costs, import charges and marketplace fees.
If the company is one of the US importers of record, customs and freight records should be matched to the accounting system. This helps prevent overstated profit and creates a clearer audit trail for imported inventory.
The business should then review the current-year tax calculation and determine whether an estimated payment is required. The final amount must come from the applicable IRS calculation, not from a rough percentage of gross sales.
This is where an ecommerce accountant UK-based businesses can work with needs to understand both marketplace data and US compliance records. The same applies when you need an Amazon seller accountant UK, Amazon FBA accounting UK or Shopify accounting UK process that connects UK bookkeeping with US reporting.
Submit QI, WP or WT applications by 30 September
The IRS has confirmed that applicants seeking an agreement effective for 2026 must submit applications through the Qualified Intermediary, Withholding Foreign Partnership, Withholding Foreign Trust Application & Account Management System, known as QAAMS, no later than 30 September 2026.
This deadline applies to:
