Book standby seats at super low prices. Standby seats are unconfirmed seats which may or may not be accepted by the operator. Or book a confirmed seat if you want surety.
The operator accepts (or cancels) the standby booking, usually around 2-3 hours before departure. If accepted, your booking is now confirmed.
When confirmed, the website automatically takes payment and sends you the voucher. Just make sure you can get to the trip in time. Show the voucher to the operator and away you go! Simple.