I would not expect a retailer to be morally bound to honor a mistake in price that was likely made by a printer or a web tech. Additionally, they corrected the mistake quickly; in less than 24 hours. Moreover, you suffered no harm.
Someone likely left off a number.
50 Rounds per Box $9.39 is incredibly low priced. The lowest price I could find today was .45 ACP 230gr Full Metal Jacket 50pk for $17.50 at Georgia Arms.
NRA Life Member - Orange Gunsite Member - NRA Certified Pistol Instructor
"When plunder becomes a way of life for a group of men living together in society,
they create for themselves in the course of time a legal system that authorizes it and a moral code that justifies it." Frederic Bastiat