Sessions are charged at the start of every new session, and every following hour. If you stop a premium bot and restart it, you will be charged a startup fee regardless if you used an hour in your previous session.
This is so users don't just stop a bot at 59 minutes to avoid paying for the hour. Try to minimize the amount of times you restart a bot during an hour--most premium bots should allow you to change settings without restarting the bot. And if it keeps crashing or something, don't use it & feel free to post on the bot's thread requesting a refund at the author's discretion.