Is it true with a flat rate envelopethe weight does not matter?

the pric only varies with the location you are sending to?

Part of vb answer is correct in that you must maintain the integrity of the flat rate envelope in order to mail at that rate,meaning you can't cut it in half and tape one half to the top and the other half to the bottom and have like and inch of space in between. Now the rate issue. $4.60 is the rate for Priority mail that is not over one pound. That rate is the same to any domestic address. 2 pounds or more the rates are higher and vary depending on how far it is going. The flat rate is a special Priority box that states Flat Rate on the box. That rate is $8.95 to any domestic address regardless of weight. Say from NY to CA a 4 pound Priority piece ( not flat rate) would cost $11.10, but in the flat rate box it would only be $8.95. The flat rate is a good way to go if you have something that is small but heavy, especially the farther away it has to go.

A flat rate envelope that still has integrity (same shape, no bulges, not help together with tape) is just that a flat rate. Some flat rates are $4.60.

The only requirement is that you be mailing from one location in the US to another. There are no price variations (that's the whole point of a flat rate!)