Rez you gotta consider a few things.
When looked at the site structure and AI of your site you have to think about the Juice flow as a funnel. More Juice to the top distributed less juice to the bottom. So for shopping cart pages or product pages ( depending on how deep they are ), i usually incorporate Long tail , targeted keywords ( ie: Mimi Juie baby sippy cups ) where the volume is not much but its targeted enough that even with a limited juice flow you can rank.
My initial suggestion to you was to contact the person or company that built the shopping cart in order to remove the link. ( THAT IS MY FIRST OPTION ). I would not do a no follow to the product page. ( dont do anything crazy like that ) Specially if you have Share bar options for your products and reviews etc. ( you will lose all that ) .
LAST OPTION for you should be to do a robot.txt to ONLY that link, NOT the page.
Again please understand you should not DEVALUE your page like that .
Hope this helps.
Let me know how it turns out
Hampig M
BizDetox
if you know what i mean .