As far as I'm aware, you dont pay a deposit. The lift passes arent the plastic card type you now get in most european resorts.
They have a bar code on them which has to be inserted into readers at the entrance to each lift. This is where you have to be careful you dont loose your pass as you do have to keep taking it in and out of your pocket. So its worth getting a lift pass holder.
Don't think that's right Hilly - certainly Pamporovo have been using scanner passes for years now, so no need to remove it from your pocket at any time.