Post-Office-Box? (Seville)

Hi!

I am currently looking for another apartment and would like to make sure that my mail doesn't get lost in the process. At my current place, the mailman puts all mail in a pile on the stairs, ignores the mailboxes, and only advertisements ever end up in the boxes. Everyone sorts through the envelopes, takes their mail, and leaves the rest on the stairs. When someone moves and mail still ends up here, it gets tossed. The other day, I came home, and various letters were lying behind the entrance door, on the floor. I almost stepped on them. I just googled for renting post office boxes here in Seville, but I didn't come up with anything. Any ideas or suggestions? I haven't been to a main post office, yet, to complain or to enquire about p.o. boxes for rent, because I work a lot and haven't had the time. A colleague of mine discouraged me from complaining at the post office, because he had done it, and it made no difference. At his building, albeit in another part of town, its the same situation.
Thanks! - Louise