I was recently amazed to learn that nearly a third of shopping cart software/programs have security holes. The security holes allow for hackers to alter prices.
According to the recent article by Laura Lorek, of Interactive Week, "Here's how it works: After choosing a product and receiving pricing information, a hacker can use a standard browser's "edit page" feature to show the hidden HTML code on the page. The thief then saves the page to his computer, alters the price information and then hits the "publish" key on the browser. In many cases, that page is then accepted by the shopping cart software - and that $999 watch becomes a $3 special."
To read the article in it's entirety, click here.
I can think of a couple approaches for fixing the problem, but that discussion is best left in specific technical forums.