First Class Large Envelope Vs First Class Package Shipping

I am knowledgeable on the difference between a First Class Large Envelope and a First Class Package. However, on auction sites, a lot of sellers display USPS First Class as a shipping method vs package for shipping modern sheets with cardboard backing.

What would happen if an uninformed seller shipped a few cardboard backed sheets in a FC large envelope? Would the USPS intercept it and charge the recipient postage due? How would this rigid large envelope get handled vs a package?

A letter as far as I know goes through the mail stream and goes around all the bends and corners on that little conveyer belt, so it by regulation cannot be unplexable (that's the word the USPS uses). A package can be as solid as a rock and they just charge you by weight, hence the higher base price for shipping. A large letter starts at $1.30 I believe and goes up by weight, and a package starts at $4ish I believe and goes up by weight and destination. The USPS is honestly in no apparent rush to collect postage dues as I have (sadly unknowingly) been under franking my own large envelopes, just putting a $1.40 on any regardless of weight since I don't weigh before I ship. Never been an issue. Are you sure they're using a "rigid" cardboard? More likely it's a semi rigid deal, like me using manila folders to wrap up items in large envelopes.

If what you wrote is true, then you understand that the difference between and envelope and package is the thickness, specifically over 1/4 " is a package. Now there are sellers who purposely make the mailing more than 1/4 " thick in order to receive the tracking offered on 1st Class Packages.

Additionally 1st Class letters (up to 3.5 oz) and 1st Class Large envelope which do not meet the machinable standards are charged a "non-machinable" surcharge to avoid the machines and must be so marked.

As to where inappropriately paid matter is marked up for postage due, it is likely the clerk accepting the mailing or the employee making the final delivery to you street or PO Box. In the mail stream, it is unlikely to be noticed unless it sticks out like a sort thumb. Two such example who be a rustic metal post card dropped into a drop box and a legal sized envelope address and stamped not in the normal "long way" but such that the top and bottom edges are the narrow ones.

I mail many "flats" and 2 oz envelopes. As long as you abide by the weight and thickness rules, I've seldom had a problem. I have some tips though. The key is for flats or even letter sized envelopes to easily slide through the automatic canceler. Use good clear packaging tape. Tape the back flap on the envelope large or small. I even use tape to go over the top or bottom (steer clear of the stamps of course.) I also long ago got one of the larger size mailboxes so flats delivered to me aren't bent! If you are really clever you can mail a fairly good-sized envelope via letter rate https://pe.usps.com/businessmail101...Name=Letters

Thank you for your comments. So from what I read, if an uninformed seller were to mail multiple cardboard backed sealed modern sheets wrapped with cereal box remnants via First Class Large Envelope, the envelope and cardboard backed sheets would likely bend when processed through the machines, correct?

If so, then I guess one has to ensure a seller is mailing cardboard backed sheets with FC Package when the shipping is displayed as First Class.

Are FC Packages treated more delicately than a a FC Large Envelope or at the end of the day, can both be "crushed" by a 20 pound box of books?