They have been doing that for awhile. Three different classes of First Class Mail - letters, large envelopes, and parcels.
A lot of people don't know that and stick any old thing in an envelope with $.44 stamp and expect it to go.
I got a check from a QB member one time and she put a little bean bag in the envelope as a "gift" to me. Ended up costing me postage due and I didn't even want it.
Here is a link to the most commonly needed postage prices. Worth your while to print it out and keep it handy if you do a lot of mailing.
http://pe.usps.com/cpim/ftp/manuals/.../notice123.pdf