If your customers are having trouble adding items to their carts the culprit is usually frames.
Many times the merchant did not knowingly add frames but their domain name registrar is adding them because of how a domain name is pointing at the store. If you have asked your registrar to use url redirection with masking, in other words point www.domainname.com to domainname.storesecured.com and hide where its actually pointing your registrar will use frames to do this.
Why do frames cause problems?
1) Secure sites do not look secure when using frames because all of the different frames will not be secure, thus your customers will lose confidence.
2) Some session cookies do not work in frames and thus some customers will be unable to add items to their cart.
3) Search engines hate frames and many have a hard time listing sites which use frames.
4) Older browsers do not support frames.
How can I tell if I am using frames?
Come to your site using the url that your customers are using and in the browser window select View-->Source. If you are using frames you will see the words frameset.
How can I correct this situation?
1) Dont use url redirection, instead please point the domain name at your site the correct way. Go to General --> Domain Name and enter your domain name and then update your nameservers to read ns.rackspace.com and ns2.rackspace.com
2) If you absolutely must use url redirection do not ask your domain name registrar to mask the url in addition to doing the redirection.
How do I know its corrected?
48 hours after making the above changes again go to your browser window and select View-->Source and the frames should be gone.