DanielT, that's beyond my knowhow. It may not be possible because the checkout is not modifiable. We do free samples that we ship for free, so I did something similar only based on $0 price ($0 price = $0 shipping). There is still a second step, though it does note on its own that it ships for free.
So I leave this one to the real experts, but still predict is cannot be done. Good luck!