What exactly is considered a USPS first class large envelope

[jamcrackers]

I have been using poly mailers I bought that measure 13x10 to send tshirts and stuff.  I listed some leggings today and a choice offered was USPS first class large envelope which would cost $3.75 but in my poly mailer on usps ground starts at  $6.15.  I am just not sure what qualifies as a first class envelope.

[kensgiftshop]

That would be an envelope.

A poly mailer would be a package.

[wooden_flower · Accepted solution]

A Large Envelope is a rectangular mailpiece no thicker than 3/4 inch. They are commonly referred to as “Flats.”  Flats must be:

• Flexible
• Have four square corners (or finished corners not exceeding a radius of 0.125 (1/8) inch.
• Uniformly thick.
• Unwrapped, sleeved, wrapped, or enveloped.

https://faq.usps.com/s/article/What-is-a-Large-Envelope-Flat

Most clothing, other than something like a scarf, will be too thick to be a large envelope.  Also - there is no tracking on a large envelope, which is why it is so much less.