Oh and my Spanish sucks and besides I was in Barcelona so my Catalan is nonexistent. But the post office personnel (have I mentioned I LOVE them?) helped me navigate the mailing system so be brave--as the hotel or albergue to point you to the post office and off you go. It seems daunting at first (if ones Spanish sucks, as mine did) but it's really the same as mailing stuff in the US (but less surly). After a few trips to the post office along the way, it was easy (although my Spanish still sucked).
As to why I love the post office: I planned a rest day approx every two weeks, in a large city I wanted to explore for more than an hour or two. I mailed ahead a care package of things to 'make my day' (favorite hard to find candy or a small soap or perfume sample, notes friends and family wrote for me, etc) for my luxury night in a hotel, as well as my 'emergency stash' of spare charger cord and socks, and 'refills' of foot ointment, etc. On my rest day I took out a treat, dipped into emergency and refill stock as needed, and sent the shoebox sized package on to the next "rest day" post office (and eventually to storage in SdC). I'd also buy stamps for the next two weeks of post cards, and refill my SIM card. Years later, even walking into a US post office makes me think about how happy I was picking up my "care package," and stamps and 'minutes' needed to reach out to loved ones.