Stamps.com does not make it easy to find, but any USPS shipments you send can be adjusted or credited automatically based on the weights and dimensions of the package. We ship a decent amount using Stamps.com via ShipStation, and in the last 2 weeks have had 2 "adjustments" to First Class shipments which were meritless and outside the stated Stamps.com policy of 30 days. The first was an 8oz First Class package which was shipped July 13th and delivered July 17th. On Sept 3, stamps.com slipped in a $32.XX adjustment that states the envelope was scanned at 19"x19"x10" 22lbs. On Sept 5, we received an adjustment for $62.79 for basically the same thing, a 0.08oz First Class package in a small padded envelope adjusted to 19"x19"x10" 22lbs.
If you ship via stamps.com be sure to check your adjustments by looking at Reports -> Balances and checking out the transaction detail. They don't make it easy to view these and it's rather shady that when you ship via ShipStation (same company as stamps.com) you don't get any kind of notification of these adjustments…. Most would have no reason to even log into stamps.com when creating shipments on ShipStation and if you don't do that, you would never even know they are happening.