Isla SMP is a fictional server. The store is real: what you buy here is a real order against a real KantoPay shop, and the pesos land in that shop's wallet. Prices are kept tiny so anyone can try the whole flow for the cost of a piece of candy.
Every call this page makes is listed under Watch the calls, each with a link to its page in the docs. View this page's source to see how little there is to it.