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.
• Use USPS packaging that doesn’t say “Flat Rate.”

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

Learn more about Ground Advantage.

If you’re using First-Class Mail (Letters)

First-Class Mail Letters are 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 bubble mailers — letters must be flat and machinable.

Learn more about First-Class Mail Letters.