You can cancel but the longer you wait, the more they charge as a penalty. I canceled once and was charged $20.
I originally used Flex pay for MJ. Later I found that Toywiz was selling her cheaper (no tax, cheaper shipping), so I called up Sideshow to switch from Flex pay to full price. They gladly switched and I cancelled it a few days later. This was all a month or so before any Flex payment would of started though.