Does it work that way? I always assumed it was just prepaid for a standard letter weight. If they charge afterwards based on weight, this could get interesting.

Kinda, but not like those that are having wet dreams over it, think.

You cannot send bricks, bolts, lead plates. But you can stuff it with paper, or other junk mail, and they will be charged $4.34 for an overweight letter. You only need a few ounces to trigger that.