I think I'm one of the only people who've seen salsa mix. (Never bought one, tho')
I've seen it at the annual Harvest Festival (which this year changed it's name, so I can't remember it's new name), where over 100 vendors rent booths in an indoor forum, and sell their goods. It's anything from crafts to foods to blown glass to woodworking. (It's on a rather grand scale, and it travels into 4 states) Anyway, it's the only place I'll pick up a mix, as I can talk with the maker, and I can taste the items before purchasing. Most items I purchase are jarred goods, like cherry butter, gourmet peanut butters, lemon curds and hot pepper jellies. But, sometimes, I'll pick up those sour cream-type dip mixes, as those are always a fun thing to have on hand.
(and, I can't imagine ever purchasing a guac dip mix, sorry!)