It took us some time to identify the problem today, which is related to your SEO module and how it handles sessions - the period when someone is visiting your site.
When a product is added to the cart and the customer clicks Continue shopping the item added is being kept in session so when a new item is chosen it is being read as the original product in memory. Unfortunately, there is nothing at all we can do to rectify this conflict between Ultimate SEO urls and ZenCart other than turning OFF your SEO module - which we have done.
However, there is a commercially available SEO module which we believe will HELP resolve your problem because it works in a different way to the Ultimate SEO module. However it will still need additional work by our programmers (about 2 hours) over and above the installation. You can purchase the installation here -
***Link Deleted***l - and we would invoice you separately for the additional coding work.