Create/Edit Plugin for Custom Hotel Booking

I'm designing a website, but need coding/developer help. My client has 10 motel rooms with number combinations as the lock, and we would like an email to go to the customer 1 hour before check-in time. I'm currently using a plugin called HBook, and this plugin sends an email to the customer right away as a confirmation that they purchased a room. But we don't want the unlock code to be sent to the customer right away, especially if they are booking a few weeks or days in advance, while other people will be staying in the rooms. And, the unlock code will change after people check out during that time period. So, basically, I need a third party "autoresponder" or some kind of creatively coded solution that would recognize the end-user's checking date and time, and send them an email with the unlock code 1 hour before checkin time.