Scummy businesses sure exist but you would never see this in Amsterdam or Paris. I'm pretty sure there are laws that whatever price is shown is the final price.

You're probably right. But judging from other comments in sounded like a small note on the front cover of a menu saying 'there will be a flat rate surcharge' would not be adequate.

Obviously, this is a horrible way for me to collect legal advice, but would a fine-print note on a menu fly? (interpret 'fly' however, I'm clearly naïve here.)