What is a first class letter?

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