I put on special offers from time to time, just make sure to stipulate a timeframe or condition (like advance booking with deposit, books through AW only or any other behaviour you would like to encourage). You'll still get the odd one who thinks he should be entitled to the discount despite being outside the timeframe or whatever, no matter how clear you make it that this is a limited offer.
For 15min appointments, I would be upfront about the limits there too (one pop, no anal etc) I make a Spotify playlist 25mins long, I press play when they call to say they've parked up and I know which song plays at 20mins so I start wrapping up then to have them out by the time the music finishes. It's a great way of keeping track of time without clock watching
xx