There is no excuse to overcharge buyers just because the PO staff might try to charge you wrongly.

I wasn't talking about PO staff try to charge wrongly.

As most of us know they use letter gauge to check the thickness of the packets/envelopes.

Some staff will push it through and make it as large letter whether she/he need to force it though or not.

Some staff say "no" to the packet/envelope when she/he needs to push it through by force even if it does go though the gauge.

And some staff just measure it by their eyes and don't check it. And the are not always correct.They do make a mistake.