The new thing is the "large envelope", which meets this description:
*Height between 6-1/8 inches and 12 inches
*Length between 11-1/2 inches and 15 inches
*Thickness between 1/4 inch and 3/4 inch
Anything bigger, in any dimension, is a package.

In addition, a large envelope can't be completely rigid, irregular (nonrectangular) in shape or irregular in thickness. Those mailing envelopes that are thin on the edge and thick (with padding) in the middle are ok.

A 1 oz large envelope cost $.83 from Chicago to anywhere in the US; a 2 oz one costs a buck...each additional ounce costs $.17. I checked Chicago to Miami, Anchorage and Guam to be sure.