How does the interstate commerce clause allow a mandate to buy health insurance?

If I have no insurance I'm not involved in commerce and when I need health care I pay cash in my state which doesn't cross state lines and therefore does not give federal government jurisdiction over my transaction.
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The individual mandate is NOT legal, in that it exceeds the power of Congress granted to it by the US Constitution. If they can force us to buy health insurance, then they can force us to buy anything they think we should own. The idea that this is covered under the interstate commerce clause is ludicrous... the fact that the federal government regulates milk prices doesn't mean they can pass a law requiring us to purchase milk. The objections to the individual mandate are three-fold. First, because federal law prohibits us from purchasing health insurance across state lines, then it is NOT INTERSTATE commerce and therefore cannot be regulated by Congress under the interstate commerce clause. Second, even if it WERE regulatable by Congress, NOT buying health insurance isn't commerce of any sort, and therefore purchase of it cannot be mandated. Third, if Congress can mandate the purchase of health insurance, then they can mandate the purchase of any product they think we should own... such as a GM car or low-fat food.
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