Why automate story replies?
Stories get quick, casual replies, and most of them ask the same thing: the link, the price, where to buy. Answering each by hand means some wait hours and some never get an answer before the story disappears. An automatic DM answers every one while the story is live, which is exactly when people are most interested.
Story mentions are free word of mouth. Thanking people straight away, and offering something back, encourages more of them.