From the ALSA.com website on the transportation of bicycles:
Transporting bicycles, surfboards or skis has an additional cost, 5 euros for short-distance routes and 10 euros for long-distance routes.
In international services, you are only allowed to carry on bicycles in a Spain-Morocco route and Spain-Portugal route, by booking and paying for this extra service.
Because of the limited space, we admit 4 objects (bicycles and surfboards) total in each coach, one per ticket. They have to be good conditions to travel without causing any damage to other baggage, so it is obligatory for them to be well packaged in boxes or bags suitable for transportation.
You can buy your ticket on our website along with bicycle transportation, indicating it in the box in step 3 of the purchasing process. Tickets with bicycle transportation must be bought one by one, so it's impossible to check this box if you chose 2 or more seats.
If there is no space in the selected coach, this box will be unavailable too.
Once the ticket is bought, the passenger must be there at least 15 minutes before the departure time with the indicated bicycle or surfboard.