We stayed in Venice and used the buses from Piazzale Roma to and from Marco Polo several times to meet family etc. The ATVO (blue buses) cost 3 euros and run every 20 minutes. The ACTV (orange buses) are the public buses only cost 1 euro but no luggage space and they stop several times between airport and Piazzale Roma. We found the blue buses very efficient and easy to use. You can buy tickets at the ATVO office in Piazzale Roma (not from ACTV office who also won't help you) and at the airport on the bus.