If it doesn't say in your manual to use your Aircon once a week or it will break, then that's a very poor excuse. I would not have paid anything from the start. Also, if this is a problem that started before your warranty expired and it still is a problem, then I'd not pay anything to get it fixed, at all. It's always tricky once you've paid something as your dealer will possibly think you're going to just accept whatever they tell you or charge you for, and also that you've accepted some kind of liability, which is never helpful to you in this sort of situation.