$0.83 is for one ounce first class "large envelope (flats)", and $1.34 is for two ounces first class "parcels (packages)."

You didn't say if the post office clerk weighed each one.

You can have a lot of fun reading the physical standards for flats v. parcels:
http://pe.usps.com/text/dmm300/101.htm#wp1039555+

Some of the things they say about flats is as follows. You can see why clerks can't spend their life learning this and doing all these tests. People in line at the post office would scream about the delay. This is only part of it:

Minimum Flexibility Criteria for Flat-Size Pieces

Flat-size pieces must be flexible. Boxes—with or without hinges, gaps, or breaks that allow the piece to bend—are not flats. Tight envelopes or wrappers that are filled with one or more boxes are not flats. At the customer's option, a customer may perform the following test on their own mailpieces. When a postal employee observes a customer demonstrating that a flat-size piece is flexible according to these standards, the employee does not need to perform the test. Test flats as follows:

a. All flats (see Exhibit 2.3a):

1. Place the piece with the length parallel to the edge of a flat surface and extend the piece halfway off the surface.

2. Press down on the piece at a point 1 inch from the outer edge, in the center of the piece's length, exerting steady pressure.

3. The piece is not flexible if it cannot bend at least 1 inch vertically without being damaged.

4. The piece is flexible if it can bend at least 1 inch vertically without being damaged and it does not contain a rigid insert. No further testing is necessary.

5. Test the piece according to 2.3b or 2.3c below if it can bend at least 1 inch vertically without being damaged and it contains a rigid insert.
Exhibit 2.3a Flexibility Test—All Flats

Uniform Thickness

Flat-size mailpieces must be uniformly thick so that any bumps, protrusions, or other irregularities do not cause more than 1/4-inch variance in thickness. When determining variance in thickness, exclude the outside edges of a mailpiece (1 inch from each edge) when the contents do not extend into those edges. Also, exclude the selvage of any polywrap covering (see 301.3.3) from this determination. Mailers must secure nonpaper contents to prevent shifting of more than 2 inches within the mailpiece if shifting would cause the piece to be nonuniform in thickness or would result in the contents bursting out of the mailpiece (see 601.2.3)

.

Robert

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