I will try and explain about cookies and cashback, say you visit QVC via a pop up or advert on a site called "SHOP" , it will place a cookie on your pc saying that "SHOP" refered you to QVC so they get the comission if you buy anything, these cookies can last up to 30 days, thats why you need to clear you cookies before you go through a cashback site as it will use there cookie instead of the "SHOP" one because the "SHOP" one was there first they still get the comission
Once you have visited you can delete the cookie it dosent need to be kept on your PC anymore
I dont bother with Quidco anymore as a lot of sites have left there, i now use Cashback Kings or Top Cash Back