I've been using finishing salts for years and love them...
For starters, do you use coarse sea salts in your cooking? Coarse salt is wonderful and you use much less. A finishing salt is what you use at the end of the dish to add a bit of interest, color and taste. You only need a grain or two on say a tomato, or a few on eggs, meat, or whatever you are presenting to be eaten. When I bought a finishing salt, I chose the pink, coarse salt, because it was pretty. Then I tried a smoked salt, then a grey, and so on.
Just as with oil and vinegars, it is a matter of playing to see what it is all about and how you like the taste of a certain salt. It is a fascinating and fun thing to do.
Stay away, from the black lava sea salt unless you like the taste of charcoal. It leaves a residue on your fingers and can leave ugly black residue on you food. If you want to try it, do so after you experiment with the simpler, more popular ones first. You might consult with the person taking your order, let her know your cooking style, what types of foods your family likes and I am sure she will be a huge help.
Good luck.
For the record, I have in my supply right now, a pink, smoked, truffle flavored (OMG) and one mixed with herbs. All very fun!