I'm looking on the Barcelona public transport system and it seems to be cheaper if one buys the card online. Here is their website.
I have two questions in this case:
How do I get the card, will it be sent in some way? I'm not in Spain now, I want to use it when I arrive there next month.
How the availability will be set? I want the 72 hours one. Will it start the time countdown from the first time I use it?