grape juice

Question

Rabbi, After having read your responses concerning grape juice, I have the following questions: if pure grape juice, packaged and bought in a Supermarket is OK, then why is it that certain juice flavors that includes grape (Tropicana orange juices containing grape, for example), do not have a kosher seal? The other Tropicana products do have a kosher seal, meaning that the company is under supervision but is not given a kosher seal for their juices containing grape. If I go to a supermarket outside of the US, and I see cartons of grape juice made by the same company -- one has a kosher stamp on it and the other doesn't -- can I buy the one without the kosher stamp, which is much cheaper? On a tangent, is it important to make Havdalah with grape juice or wine, or is tea or orange juice just as good? Thank you very much.