Hi Transalp,
I have had relatives travel down to Tortosa by Bus recently.They paid in cash (for their tickets) on the actual airport bus.It seems the airport bus (from El Pratt) maybe mainly a shuttle service into Barcelona,where you get on another bus taking you on the AP7 motorway down to Tortosa. On their actual journey down,they had to change again just outside the Motorway exit near L'Aldea.I think the changes all depend on how many passengers are destined for Tortosa.
When they went back,they only had to change once.
Hope this helps.