How do I avoid problems when I'm trying to get back to the airport after a bus trip?

I'm planning a solo trip to Peru and want to go on a bus from Lima to Cusco and then Machu Picchu and then back to Lima. The buses are overnight, so I don't lose a day to the trip. However, I'm a little confused about how to get back to the airport to catch my flight back home. I have a return flight that I'm not cancelling and the flight is booked already. <br><br>The flight is from Lima to Chicago and leaves at 10:30 PM. That means I have to be at the airport by 7:30 PM the latest. Does the bus company sell tickets that guarantee you arrive by a certain time? That would be a stupid thing to do, but I don't know another way to ensure I can get back to the airport and not be late.<br><br>I'm not going to rent a car, because I don't speak any Spanish besides basic phrases. If the bus breaks down and I'm late to my flight, I don't know what I'd do. I'd rather be safe than sorry, and don't want to take a bus to Cusco and then Machu Picchu and then back to Lima and miss my flight and have to pay extra for another one.