Has anyone attempted to do Andorra on a day trip from Barcelona? I did a search but haven't found good concrete information yet.

What bus station/line do I use? Also, I hear Andorra is mainly just good for duty free shopping, but my main reason for wanting to go is just to say I've been and maybe pick up some trinkets, so that's the reason for me only wanting to stay a day.

Any ideas or help?

Andorra as a daytrip from BCN? it's not worth the effort. It's far for a daytrip, plus, depending on the day you make it the traffic could be horrible.
But there are other things in Andorra appart from shopping... it's in the hearth of the Pyrenees, it has the best skiing of the whole range, and it has great romanic monuments and the landscape, of course.

Barcelona to Andorra is not a good day trip... It takes 3 1/2 hours by bus. There are no train stations in Andorra proper. By car it takes about 3 hours barring traffic and depending on how you drive in the mountains...