So, I assume that they want you to pay for an additional 2 days at the daily rate, to pick up some extra cash.
I'd talk with the owner, and express your thoughts about this, and let them know that you are displeased. They may give you a break, if they thing you will be a customer next season. If not, I'd let them know that you WON'T be back next year, and I'd leave on the 8th....I doubt that they will make any extra money, and I'd sure as heck not be the one giving it to them!
Is it a common practice.....maybe, though it would sure be nice, if they put it in writing someplace (which they may have).....be glad that they didn't use February as their guide...you'd be leaving a week early!!:)