The only way to buy MC is through Steam (even though the client says purchase MC with your local currency on PlayHawken.com) and Steam only accepts PayPal or credit cards.
Allowing people to buy MC using a system like OpenBucks or Rixty would help those who can only buy it with cash or would rather buy it with cash. Also it would be a good idea to change the text in the client that says you can buy MC on the website.