Fix checkout cart for a drupal open deals distribuiton

I have a drupal opendeals distribution website that need to have some configuration to the checkout cart. The problem I have is that I can not put multiple items into the cart. I can only put one item in the cart and if I want to add another item, the original item would be deleted and the new updated item would show into the cart. I would like the cart to be able to add multiple items without any item removed. You can test the checkout cart from our website at www.smurfi.com. (test it by trying to add 2 different items to the cart, then checking the cart to verify whether they are still there) This job requires drupal skills that are necessary to complete the project. You must be an expert in Drupal and the OpenDeals distribution and must have knowledge in doing the job, or have done so in the past. Drupal uses a blocks system that you must have knowledge of configuring. Here is the link to the open deals distribution available on drupal: [login to view URL]