Rohirrim do not come, instead Helm's Deep is helped by average guy with access to Amazon and unlimited fund, how does he holds the siege?

The guy has laptop/phone with amazon, infinite money and the purchase is immediately appearing near him in safe space.

King Theoden is fully supportive as are his people, trying to rely on this weird man. Aside from that the guy has to use his smarts and creativity to make Helm's Deep as unwinnable battle for Sauron forces as possible, hoping that everyone survives.