Gail, Sorry to say that not all RAM Manufactures off 100% Lifetime Guarntee and 100% Replacement for Bad Ram.
A while back there was so much innconsistancy in Ram that some of the makers started this to boost sales and they improved there product that they felt it a must because of the ones that were not so good.
As luck would have it you can go to..
http://www.macmall.com
I think if you look around there you will see some do and some don't.
Apple uses different suppliers for there products, and it is hard to tell which brand was used in your iMac,
I doubt that Comp USA will stand behind it but it is worth a try, Ram is cheap right now and you might check to see if there is a page available in the fourms that give a breakdown on replacing it in your machine..Good Luck..Don