Is Marketpress conflicting with simple paypal shopping cart plugin?

My site is It is a membership site that also sells an ebook using a simple paypal shopping cart plugin (Simple Paypal Shopping Cart Settings v 3.2.7)....
Just added marketpress a few days ago, and more book sales:slight_frown: After seeing such a drastic lull in book sales, I investigated.
When someone tries to purchase my book (, the page is redirected to and says the cart is empty.....sooooo, it IS conflicting with marketpress. How can this be fixed?