Packaging rules for USPS labels

Using the wrong box or envelope with a USPS label can cause delivery delays, extra fees, or lost packages.

This article explains what packaging you can use for each shipping option.

If you’re using a Flat Rate label

Flat Rate offers a fixed shipping price based on the package size, not weight or distance. Delivery is usually 1–3 business days.

You must: 

• Use USPS-branded Flat Rate boxes or envelopes.
• Avoid using your own packaging.
• Avoid using Flat Rate packaging for Priority Mail, Ground Advantage, First-Class, or Media Mail labels.

⚠️ Look for “Flat Rate” printed on the packaging. If it doesn’t say “Flat Rate,” it’s not eligible.

Learn more about Flate-Rate packaging.

If you’re using a Priority Mail label

Priority Mail offers faster delivery (1–3 business days) and includes up to $100 of insurance.

You can:

• Use your own packaging.
• Use USPS Priority Mail boxes or envelopes that don’t say “Flat Rate" (example box).

⚠️ Don’t use anything labeled “Flat Rate” unless you have a Flat Rate label, even if it says “Priority Mail.”

If you're using Ground Advantage

Ground Advantage is a lower-cost shipping option with delivery in 2–5 business days. It includes up to $100 of insurance for commercial users.

You can use your own packaging.

⚠️ Don’t use "Flat Rate" packaging with Ground Advantage labels. 

Learn more about Ground Advantage.

If you’re using First-Class Mail Letter

First-Class Mail Letter is for lightweight flat envelopes. Delivery is usually 2–5 business days.

You must:

• Use a plain white business-size envelope with a security liner.
• Avoid using padded envelopes, bubble mailers, or rigid packaging — letters must be flat and machinable.

Learn more about First-Class Mail Letters.

If you’re shipping to international, U.S. military, or U.S. territory destinations

International, military, or U.S. territory shipments generate three separate pages for labels instead of one. All three pages must be attached to your shipment. 

All of the required information will come pre-filled when you generate your label, so you should not have to add any additional information to these labels yourself.

To see what this label format looks like, please click here.

You must:

• Attach all three pages to the address side of the package.  We recommend using USPS Large Customs Declaration Form Pouches, which are available free of charge by USPS.  You can put all three labels inside the pouch and attach it to your package.

Learn more about shipping internationally on Whatnot