Here's the info I got from my post office. (it's been an ongoing learning lesson)
There are 3 weight classes for the 'small' package.
For simplicity sake, let's call them small, medium, and large.
Small= regular flat envelope
Med= thick envelope under 1/4 inches thick
Large= package, over 1/4 thick
(these all are uniformly the same thickness. If an envie is NOT uniform in thickness, there is an additional charge of 20 cents)
the following prices are for under the ounces listed
>1oz
sm= .44
m= .88
L= 1.22
>2oz
m= 1.05
L=1.39
>3oz
m= 1.22
L= 1.56
>4oz
m= 1.39
L= 1.73
>5oz
m= 1.56
L= 1.90
etc, etc.
Don't forget! If it's not uniform in thickness, add 20 cents. If it's not the correct shape, add another stamp. If it's non-machineable for any reason (fragile, etc) add 20 cents.
Most things weigh less than what we think, however, they get us on all the details (shape, size, etc) all that costs additional $.
I picked up a cheap scale in the food storage section at WallyWorld. It gives the ounces.
Still, I will sometimes stll take them to the PO to verify (so if I'm one of the culprits, PLMK!!!) My post office is nice, and will often try and slip my packages into the lower category.. I'd hate to think anyone is getting charged for this reason!