I am becoming a Iron supporter but only for one month. Do I have do push a button to stop the supporter or what?
On the upgrades page just hit the cancel subscription button near the top as soon as you've completed the payment. That will stop it renewing.
Basically what Jack said, but you will keep the perks until the next month. Say you bought supporter on July 4th, you would have it until August 4th.
So when I pay and done all the forms I click the cancel subscription button and I will have the membership for 1 month.