Not true - at least not with state commerce (within it's borders, as long as it does not affect interstate commerce). State can regulate product purchases, etc. This is not regulating the money, but rather the commerce (intrastate). Two different things.
There's a famous case where the supreme court found that a law telling a farmer how much grain he could grow FOR HIS OWN USE was inTERstate commerce, because in aggregate it affects national commerce.
Here we have advertising/solicitation on TV and the internet, both of which are interstate media, a company with national presence and stores in many states, probably incorporated in one state and headquartered in another, selling phones to use on a phone network that crosses state borders and which is regulated by the FCC, and the issue comes down to whether federally printed and issued currency can be used or whether credit cards, which are issued by federally insured and regulated banks and which have their transactions cleared on interstate or international banking networks, can be required.
Given the case law, this particular case seems pretty interstate to me. Imagine the effect on interstate commerce if no one could require identification, credit checks, etc. prior to engaging in a transaction.
I'm willing to place a friendly wager on what happens, if you'd like
Update: The case to which I referred:
http://en.wikipedia.org/wiki/Wickard_v._Filburn
If you don't like wikipedia, I can PM you a copy of the actual decision by the court.
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