Could have been a parasite or more likely something to do with the water quality of the LFS
coupled with being transported. A parasite could kill in a few days but an otherwise healthy "looking" fish should not die on the way home from the onset of parasitic activity. I would lean more towards acidosis.
Always take the safest route though after an unexplained fish death. Since there are no fish in you tank, I'd leave it that way for the next 6 weeks unless you can rule out a parasite.