Unless things have changed you do not buy Helms Deep from Turbine. You are buying from Digital River. Digital River is a company that sets up and operates web stores for Turbine and thousands of other companies. When you go to check out, you need to create a Digital River account and provide Digital River with your payment information. Once having created your Digital River account you can buy stuff from thousands of companies that hired Digital River to set up their web stores. Some big name companies like Microsoft, Adobe and SanDisk along with number of game companies like Turbine, Electronic Arts, Square Enix and Wizards of the Coast use Digital River.
I vaguely recall that I created my Digital River account many years ago to buy something from Microsoft. I used this account to buy Rise of Isengard and Riders of Rohan.
Unless stated otherwise, all content in this post is My Personal Opinion.