Product page Add To Cart

I installed MarketPress on a client's website (www.capecodbearfactory.com). If i add products to the cart from the products list page, it works just fine (/animals/products/). If I then go to one of the single product pages (for example /animals/products/test/) and add that product to the cart, it removes any other items in the cart. I'm not sure what is going on?