That stinks, and I still can't understand the no post-it note thing myself, but I've had them insist it be removed or pay for a letter.
I've also been told by my post office that enclosing a card with a package (such as a b-day) is also a no-no when mailing 1st class. I did argue that flat-rate was advertised as if it fits, it ships. At my local po, the person working the window can vary from lovely to dreadful; some are just plain nice and others seem to want to spread their misery around no matter how nice, patient or understanding you are.
I'll close with the fact that I have issues with the po. My m-i-l worked for them for only 13 years and retired with a very nice retirement package :confused1: . Maybe my m-i-l clouds my judgment a little.