
Think of a flat as a "floppy envelope" containing a sheaf of paper or perhaps a thin catalog. Additional requirements for a flat are that it be flexible, not rigid, and with a uniform thickness up to 3/4 inch. The maximum size of a flat is 12 x 15 inches. So would an envelope measuring 8.5 x 11 inches - because the WIDTH exceeds 6-1/8 inches. For example, a 7 x 10 inch envelope would qualify because 7 inches is greater than 6-1/8 inches. Either one or the other, not necessarily both.

A "large envelope" aka flat must be at least 6-1/8 inches WIDE (short dimension) OR at least 11.5 inches LONG (long dimension). it is a PARCEL, not a large envelope aka flat. If there's any rigid item inside the envelope - cardboard, metal, plastic, etc. HOwever, an envelope that is actually 6.5 inches wide would qualify as a flat aka large envelope as long as it is NOT RIGID. No, a 6x9 envelope of any construction is too small to be a flat aka large envelope.

or 1/4 inch thickness, it would be a parcel aka package. This chart (at the end of Notice 123, illustrates and specifies the characteristics of letters, flats and parcels: A 6x9 rigid envelope would be a non-machinable letter as long as it is NOT LUMPY, no more than 1/4 inch thick and weighs a maximum of 3.5 oz. It can be rolled, folded, creased and bent. Additional requirements for a flat are that it be flexible, not rigid, and with a uniform thickness up to 3.4 inch.
The parameter might read like this "Have TWO dimensions that are greater than 6-1/8 inches high AND 11-½ inches long" If the word AND was used in the defenition then there would be a minimum lenght and width together that defined the parameters. To me the word OR says that as long as ONE of the dimensions is greater that 6-1/8 inches then it can qualify as a "First-Class Large Envelope". Be no more than 12 inches high x 15 inches long x ¾ inch thick. Have ONE dimension that is greater than 6-1/8 inches high OR 11-½ inches long OR ¼ inch thick. the USPS definition of " First-Class Large Envelope" is. I would love to use the kiosk to print postage and not have to deal with the counter staff telling me it has to ship $3.00 parcel post. Since one side of the bubble mailer is more than 6 1/8 inches long, it seems to me, that the USPS definition allows me to do this.

My confusion comes in as to wether or not I could ship them in 6x9 kraft bubble mailer with actual outside dimensions of 6 1/2 by 9 1/2. This allows me to ship "First-Class Large Envelope". I've been shipping them in 8 X 10 kraft bubble mailers with no problem (using extra bubble wrap to make it uniformly flat at about. The item I'm shipping is a small bag of nuts and bolts. There seems to be a lot of confusion about this topic on the boards as well as at the post office.
