I've been to 4 of my local post offices. I've asked them all about that cardboard slot and they told me they didn't know where to get one.
I've had them send back packages asking for additional postage but when I take them into the office I ask them why and they can't tell me why and they accept it with the original postage.
I'm very frustrated with my post offices. It seems like they don't even know their own rules.