I don't think you can stop a crazy person from acting crazy as long as they aren't acting too crazy. I think you just need to ignore her. At some point, if her behavior continues or worsens, your employer may have to take steps to keep her from coming into the place of business.
As far as what happened to spark this, I don't know what the policies are, but I would refrain from sealing an envelope for anyone else. Hand them the envelope and let them put into what they want sent. Then, if they mess it up, its on them. When its their fault and they had to pay for the unusable postage, I doubt they would be so insistent on it being "neat."
Of course that would presume that the person is reasonable which this woman obviously is not.
At least by her acting out so obviously, people know that you are not at fault.