We have many seasonal customers, who have seasonal plowing but per time salting. In the case of these it doesn't matter, if it can be melted with magic, they are charged for the salting, and we have one less plowing to do. We typically would always salt these customers after plowing anyway, so they are charged the same either way its done, and we saved ourselves considerable time.
If its a customer who is per plow and per salt charging, and we get 1.5 inches or less, we may be able to melt it all curb to curb, charge them only for a salt, and not for a plow. This saves us the time of the plowing, and saves them a little money occasionallly. However , if its 2 inches or more, even if the magic melts a lot or most of the snow, you still need to come in for a good scraping and cleanup, tucking corners, moving drifts, scraping slush, etc. This is charged as both a salting and a plowing then, the only difference is your plowing can get done in about half the time you would have spent without preapplying.