Spain's largest bus company, ALSA, has made it easy to book a ticket with a company with their new Android and iOS app.
Previously you had to find a printer when booking from the ALSA website (or from the meta-search site, Movelia), which is not convenient when on the road in Spain. But now all you have to do is book on the app and flash the digital version of your ticket from your phone to the driver to get on board.
Bus travel in Spain is often a lot cheaper than going by train. Though Spanish bus stations are often harder to navigate than train stations, the savings often make the added difficulties worthwhile. Read about Bus v Train Travel in Spain