I have a theory.
I have a theory that post offices in Spain aren't functional. I've sent simple postcards into Spain from Hungary, Switzerland, and the United States over the past two years and none of them have reached their destination, usually Valencia. I sent a box from Salamanca to the United States and the last time the status was updated on the website was 16 December. (Yes, I've resorted to checking the status of my box. It's supposedly at some international stop in unknown location, Spain. My mom even sent them an email to which they still haven't responded). Lastly, I belive that Spanish post offices aren't functional because they're difficult to find. It's very suspicious. In comparison to Switzerland, Portugal, and the US, post offices are a-plenty, around every corner. I bet you will be shocked to know that Salamanca only has one post office. It's a monster of a building, but not on my beaten path. For over 4 kilometers, I cradled that bulky box. And now I want it. Badly.
I have a funny feeling that once you reach this post office that they don't want you to send anything. You enter to a room full of people with too few seats. You get your number and fill out paperwork in front of the people who do have seats. People stare. Waiting time: normally 30 minutes plus. The cardboard boxes they offer are affordable but they're strange. You can't put the tabs in if the box is completely full but you can't fill the box all the way up or else you won't be able to close it.
Maybe my argument isn't strong, but my experience is. Ye be warn'd: if you travel to the Iberian Peninsula, take a sturdy box with you. They're free at the post office across the United States, as I just learned today...