Not mentioned in the article but I asked a shopkeeper why they use it. Answer is fraud. Earlier, customers would download a "fake" app, which miciked online gateway's UI. Customer would punch in money, and show to shopkeeper that "payment went through". Customer takes goods with them and leaves. Shopkeeper looses money.

With this sound box, shopkeeper gives goods ONLY after the box makes sound. Now imagine if a elderly or illiterate relative of shopowner is manning the shop. They may not know how to operate their "banking" app to make sure money has reached. The sound box removes that problem.