This is a very common practice. The rental company insisted you weren’t covered when you were so I suggest you complain on that basis. i.e. the desk staff’s ‘facts’ were wrong and you incurred costs as a result. They will definitely fob you off and dismiss the complaint but escalate beyond them to whichever body oversee them, they are obliged to tell you who you can escalate the complaint to.
EDIT – having read your latest reply it looks like you turned down the extra cover initially then changed your mind at check-in. They really are a bunch of shits, the extra cover should cost much less than you paid at the desk.