International First-Class Mail: Letters, Packages, and Customs

First-Class Mail International is the cheapest way to send documents from the United States to more than 180 countries, with a one-ounce letter or postcard costing $1.70 to any destination — a single Global Forever stamp covers it.¹ If you’re sending merchandise instead of paper, a separate USPS service handles small packages up to four pounds at higher rates. Which one applies depends on what’s inside, not just what it weighs.

Letters vs. Packages: Two Different Services

USPS splits international first-class shipping into two products, and confusing them is the fastest way to have your item rejected at the counter.

First-Class Mail International (FCMI) covers postcards, letters up to 3.5 ounces, and large envelopes up to 15.994 ounces. It is limited to documents and includes no tracking and no insurance.²

First-Class Package International Service (FCPIS) covers small packages containing merchandise valued at $400 or less, weighing up to four pounds. FCPIS offers limited electronic delivery confirmation to select destinations but no insurance.³

A birthday card with a letter tucked inside is FCMI. A small gift in a box is FCPIS, even if it weighs less than a heavy letter. Contents decide the service.

Size and Weight Limits

Every item has to meet specific physical dimensions. Anything outside them gets returned or reclassified at a higher rate.

Letters

Standard letters must be rectangular and fall within these dimensions:⁴

Minimum: 5.5 inches long, 3.5 inches high, 0.007 inches thick
Maximum: 11.5 inches long, 6.125 inches high, 0.25 inches thick
Weight limit: 3.5 ounces

A letter that won’t fit through automated sorting machines picks up a $0.49 nonmachinable surcharge on top of regular postage.⁵ Rigid enclosures, clasps, and uneven thickness are common triggers.

Large Envelopes (Flats)

Large envelopes must be rectangular, flexible enough to bend through sorting equipment, and uniformly thick. They can go up to 15 inches long, 12 inches high, and 0.75 inches thick.⁴ The weight ceiling for flats is 15.994 ounces, not a full pound. USPS rounds at three decimal places, so anything between 15.995 and 15.999 ounces gets bumped to 16 ounces and rejected.⁶

Postcards

Postcards must be rectangular, at least 3.5 inches high and 5.5 inches long, with a thickness between 0.007 and 0.016 inches. Cards larger than postcard limits but still within letter size can be mailed at the letter rate.

Packages

FCPIS accepts items up to four pounds.⁶ Packages containing merchandise valued over $400 cannot use this service and must ship through Priority Mail International or another higher-tier option.⁷

What It Costs in 2026

Every destination sits in one of nine FCMI price groups. For pricing, those groups collapse into four tiers: Group 1 (Canada), Group 2 (Mexico), Groups 3–5, and Groups 6–9.⁶

The one-ounce letter rate is a flat $1.70 to anywhere. A single Global Forever stamp covers it, and those stamps never expire — a stamp bought today still covers a one-ounce letter or postcard even if rates rise later.¹

Heavier letters are where the price groups start to matter:⁵

2 oz: $2.00 Canada, $2.55 Mexico, $3.40 Groups 3–5, $3.40 Groups 6–9
3 oz: $2.70, $3.40, $5.10, $5.10
3.5 oz: $3.40, $4.15, $5.75, $5.75

International postcards are $1.70 to any country.¹

Packages use a separate set of price groups running up to 20, with FCPIS rates starting at $19.40.² A country can be in FCMI Group 3 but FCPIS Group 14, so a cheap letter rate does not predict a cheap package rate.

How to Address an International Envelope

International addresses follow stricter rules than domestic ones, and mistakes here are the top reason for returned mail. The destination country name has to appear as the very last line of the address, written in full capital letters with no abbreviations.⁸ “UK” will get rejected. Write “UNITED KINGDOM” instead. Everything above the country line should follow the destination country’s local format, including any postal codes.

Your return address goes in the upper-left corner as a standard U.S. address. Using one isn’t optional for international mail. Without a return address, an undeliverable item has nowhere to go and gets destroyed.

What You Can’t Send

Federal law classifies explosives, flammable liquids, corrosive chemicals, and similar dangerous materials as nonmailable in any class of mail.⁹ International shipments add a second layer of restrictions.

Under the Universal Postal Convention, coins, banknotes, securities payable to bearer, and precious metals or gemstones cannot go in uninsured letter-post items.¹⁰ Some countries allow these items in registered or insured mail; many ban them outright.

On top of that, every country keeps its own prohibited and restricted items list. Australia bans certain food products. Germany restricts over-the-counter medications sold freely in the U.S. The only reliable way to check is the Individual Country Listings published by USPS, which detail what each destination permits and prohibits.¹¹ Skipping this check is how people end up with seized packages and, in serious cases, civil penalties.

Customs Forms

Ordinary letters containing only personal correspondence do not need customs forms. Anything with merchandise inside, or any item weighing more than 16 ounces regardless of contents, requires a customs declaration.⁷

The form asks for a specific description of each item, its weight, and its fair market value in U.S. dollars. Vague descriptions are the single biggest cause of customs delays. “Clothing” will sit in a warehouse; “women’s cotton t-shirt” clears quickly. Terms like “gift” or “merchandise” don’t qualify as descriptions.

Since September 2025, senders shipping commercial goods must include the six-digit Harmonized System (HS) tariff code for each item.¹² For a personal gift, including the code isn’t required but still speeds up clearance on the other end.

Many foreign postal administrations now require Electronic Advance Data — a digital transmission of the sender’s name, address, and package contents before the item arrives.¹³ Creating your shipping label online through USPS.com generates that transmission automatically. Paper forms filled out at the counter may require manual entry, which slows things down.

Where You Can Drop It Off

A one-ounce letter with a Global Forever stamp can go straight into any USPS collection box. Heavier or thicker items face restrictions rooted in aviation security rules. Any mailpiece bearing only stamps as postage that weighs more than 10 ounces or measures more than half an inch thick must be handed directly to a postal employee at a retail counter. You cannot drop it in a collection box or give it to your letter carrier.¹⁴ The rule doesn’t apply if postage was paid through a meter or an online postage system, since those tie the shipment to an identifiable account.

At the counter, the clerk verifies your postage, reviews any customs forms for completeness, and accepts the item. Packages requiring customs documentation need the counter visit anyway, so plan for the trip.

What Your Recipient May Owe

Postage gets your item to the destination country. It does not cover import duties, taxes, or processing fees on the other end. When a package arrives, the recipient is responsible for any customs duties, value-added tax, and postal handling fees before they can take possession.¹⁵ If the recipient refuses or can’t pay, the package may be returned to you or abandoned.

This catches a lot of people off guard when sending gifts. Most countries apply their own duty-free thresholds, and items below a certain value pass through without charges, but those thresholds vary widely and some are quite low. Declaring an accurate value on the customs form matters here: understating the value to dodge duties is customs fraud in most jurisdictions, while overstating it saddles your recipient with unnecessary charges.

USPS offers a prepaid import duties option for some services, but not for FCMI or FCPIS. It applies only to Priority Mail International and higher tiers.

Adding Proof of Mailing or Extra Security

Because FCMI includes no tracking and no insurance, senders who need proof of mailing or added security have two affordable add-ons.

Certificate of Mailing

A Certificate of Mailing costs $2.40 and gives you a receipt showing the date USPS accepted your item.⁵ That can matter for legal deadlines or contractual obligations. It doesn’t confirm delivery, provide tracking, or insure the contents.¹⁶

Registered Mail International

For genuinely important documents, Registered Mail International adds physical security within the United States. Items are handled separately from regular mail and stored in restricted-access areas during transit. The service costs $23.40 per piece on top of regular postage and provides indemnity coverage up to $40.20 for loss, damage, or missing contents.⁵ You get a mailing receipt and a delivery record kept at the destination office, but the item cannot be tracked in real time during transit.¹⁷ The recipient’s signature is not required at delivery in the destination country, so it’s better thought of as a security service than a delivery confirmation service.

How Long Delivery Takes

USPS does not publish a standard delivery window for First-Class Mail International. The official guidance is only that delivery varies by destination.¹ In practice, letters to Canada and Western Europe have historically arrived within one to two weeks, while mail to parts of Africa, Asia, and South America can take several weeks or longer. There’s no guaranteed delivery date and no refund if an item arrives late.

Recent regulatory changes have added uncertainty. As of late 2025, changes to U.S. customs rules — particularly the suspension of the de minimis duty exemption — placed new duty collection burdens on postal carriers. The Universal Postal Union reported that 88 postal operators suspended services to the United States as a result, disrupting parts of the international postal network.¹⁸ While those suspensions primarily affect mail coming into the U.S., the disruptions ripple through the broader network and can slow outbound deliveries too. If guaranteed timing matters, Priority Mail Express International is the only USPS product that offers a date-certain commitment to select countries.

1
United States Postal Service. First-Class Mail International
2
United States Postal Service. International Mail Services and Shipping Rates
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United States Postal Service. First-Class Package International Service
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United States Postal Service. First-Class Mail International
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United States Postal Service. Notice 123 – January 2026 Price Change
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Postal Explorer. International Mail Manual – Country Price Groups and Weight Limits
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Postal Explorer. International Mail Manual – 123 Customs Forms and Online Shipping Labels
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Postal Explorer. International Mail Manual – Addressing
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Office of the Law Revision Counsel. 18 USC 1716 – Injurious Articles as Nonmailable
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Universal Postal Union. Article 19 of the UPU Convention
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United States Postal Service. International Shipping Restrictions
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United States Postal Service. Postal Bulletin 22682 – IMM Revision: Harmonized System Codes
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eCFR. 19 CFR 145.74 – Mandatory Advance Electronic Data (AED)
14
United States Postal Service. Publication 52 Revision: Stamp Mailpieces Over 10 Ounces
15
United States Postal Service. Prepaid Import Duties
16
United States Postal Service. Certificate of Mailing – The Basics
17
United States Postal Service. Registered Mail International
18
Universal Postal Union. FAQ: Impact of Recent US Customs Regulation Changes on International Postal Services