@Janet53 as an experienced SH you will know this is not allowed. There is no disadvantage to a host on your adding an extra day, all they need to do is click to accept.
Is this charge mentioned on the listing?
Sadly Airbnb do let large scale airbnb management companies get away with doing things other hosts aren't allowed to such as charging deposits, age discrimination etc.
Did you book through an airbnb management company rather than with someone who manages their own listing?
In your situation I would call Airbnb and absolutely refuse to pay - it's outrageous. Personally I would ask Airbnb to cancel penalty free as the host is asking for money outside of Airbnb's terms.