Upfront spirit new page/post bug? - displaying another page

Whenever I tried creating a new page/post, they look exactly like one of my page in the website.
Worst yet, when I press the "Buy Now" button, the shopping cart page also disappeared and the exact same page I was referring to appears:
http://mynotes4sale.com/store/products/english-a1/

What is going on? anyone could help?
thanks