I recently went through quite a trek to get an extra bottle.
For some reason, the previous owner of my house left us one Repsol bottle and one Cepsa.
There is a petrol station near my house that sells Repsol, but the nearest Cepsa place is quite some way.
So idealy I would have liked to have two Repsol bottles. The neighbours told me that it's no problem, just go to the petrol station and but an extra bottle, bring the empty Cepsa bottle back to the Cepsa place and get the deposit.
It turned out neither operation was that simple.
The Repsol place said I had to get a contarct from a Repsol office. they told me the nearest office was in Alicante city, but by accident I discovered there is one in Campello which is far closer. So i went there and they modified the contract for me, but it turned out i couldn't just load up a bottle and take it home. The truck had to deliver it to me and as they couldn't tell me what time the truck would be coming this meant staying home all morning.
Then geting riod of the Cepsa one wasn't that easy either. The guy at the station said no, he couldn't just exchange it for the deposit as he wasn't allowed. I even offered to leave it for free but he said I can't do that either as it's illegal. So I actually ended up exchanging it for another full bottle to keep as a spare and thus postpone solving that particular problem.