What is the nearest train station in Spain to Andorra? What bus does one take from that train station to Andorra La Vella? Where can one get one's passport stamped in Andorra?
Many years ago (more than 40) when I drove through Andorra, the town made its living off of smuggling, postal marks and passport stamps...they've added skiing since then but otherwise, if you are not driving you will have to go through a lot of trouble just to get there and back.
...and, unless you've never seen a shopping mall in the mountains before, it's probably not worth the bother.