First-Class Large Envelopes  
There’s a whole new dimension to envelopes.
Large envelopes are great for sending items through the mail like correspondence, proposals, and newsletters. The weight limit for First-Class Mail large envelopes is 13 ounces. For heavier pieces see Priority Mail.
 
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First-Class large envelope dimensions:

Large envelopes exceed any one of the maximum dimensions of a letter.
The maximum size of a large envelope is 12 inches high by 15 inches long by 3/4 inches thick.
Mail pieces that exceed the maximum dimensions for a large envelope are charged parcel prices.

Add Extra Services:

Proof at Deposit

Certificate of Mailing – Provides evidence of mailing.
Certified Mail™ – Provides a mailing receipt and online access to delivery information.
Collect on Delivery – Allows the postage and price of an item to be collected from the recipient by the USPS and given to the mailer.

Protection in Transit

Insured Mail – Insurance against loss or damage for merchandise and gifts.
Registered Mail™ – Provides maximum security and date of delivery or attempted delivery.

Related Publications

A Customer's Guide to Mailing – A simple, easy-to-understand guide to mailing in the U.S.
First-Class Mail postage prices