I figured out the problem is due to the javascript code I used to add multiple items to the cart. The code uses the following url
http://www.demo.hikashop.com/index.php?option=com_hikashop&ctrl=product&task=updatecart&data[ZZ]=YY&data[AA]=BB&checkout=1
That will add YY items of the product with the id ZZ and BB items of the product with the id AA in the cart at the same time.
here is a page you can test that does not work
http://www.inkacollective.com/testing/consilium/products/rf-probe
However, it does work when I am using the default TAG insertion extension which can be seen at this page
http://www.inkacollective.com/testing/consilium/products/rf-probe
I am not sure how to adjust the URL to submit multiple items to the cart and also allow them to add to the same items already in the cart???
Thank You.