Thanks Wai. I'm afraid it's just as you say: I've tried with Amazon.com where I've been able to buy another kind of things in the past, but it seems that it doesn't allow to purchase videogames if you're outside the US (at least The Sims).
My despair comes from the fact that I don't even have the basic game (it's not that I would like to add some new world or an expansion or a stuff pack). And I'm dying to start playing!
I was hoping I would be able to afford the game with at least the Pets and Generations expansions, but as you've said, it seems we have to wait until we find a good chance to buy them.
Right now, in the Spanish Origin Store, the basic Sims3 game costs $55, same as any of the expansions, even the older ones. So I'm facing $165 instead of $45 to be able to start with the basics I wanted...
Maybe I should try with physical stores or ebay for physical disks.
Thanks anyway for your reply; now I know it's nothing I can easily work out, so I'll focus on finding better options.