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Found info on Cuba - Get around - Viazul:
Reservations can be made in advance on their website, but this is typically only necessary when leaving from or going to popular destinations in high season. Reservations can also be made at a Viazul ticket office (usually located at or near the place where the buses stop). If the bus is full, itβs very likely that youβll be offered a ride in share taxi for the same price as the bus. If there are no shared taxis going to your destination, the ticket salesperson will likely advise you to arrive half an hour before the time of departure and wait for a late cancelation. If there is a late cancelation, you will be allowed to purchase a 'ticket' from the bus driver.
Tips for independent travellers to Cuba:
Many routes are oversubscribed. If your travel plans arenβt fixed, turn up early at the station and join the queue.