First-Class Mail  
First-Class Mail gets your mail there fast.
Postcards, letters, large envelopes (flats) and small packages can be sent using First-Class Mail®. This service, along with Priority Mail and Express Mail, is ideal for sending personal correspondence, handwritten or typewritten letters, and bills or statements of account. It may also be used for advertisements and lightweight merchandise. First-Class Mail prices are based on both the shape and weight of the item being mailed. For items weighing more than 13 ounces, use Priority Mail.
 
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First-Class Mail includes:

First-Class Mail Cards – rectangular cardstock mailpiece not contained in an envelope.
First-Class Mail Letters – small rectangular mailpiece no thicker than ¼ inch weighing 3.5 ounces or less.
First-Class Mail Large Envelopes – flat rectangular mailpiece no thicker than ¾ inch.
First-Class Mail Packages – a box, thick envelope, or tube weighing up to 13 ounces.
Presorted First-Class Mail – for high volume business mail
Priority Mail® - Cost-effective delivery in 2-3 days to most locations.

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.

Confirmation of Delivery
Delivery Confirmation™ – Provides date and time of delivery or attempted delivery on First-Class Mail packages.
Signature Confirmation™ – Provides date and time of delivery or attempted delivery, and requires a signature of the person who signed for the item.

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