I just contacted their customer support and got the following response:
CS:
Let me double check on that for you, I believe you need to select a plan before you are able to accept payments on your store! I will just confirm that. One moment
CS:
So all you need to do is to select the plan that you would like to use on your store, you will not be charged until the trial is done!
CS:
However you do need to select a plan
Edit: oh I think I get it, you should select a plan first, fill in all your CC information and then you won't be charged until the end of the free trial period. Just before the end of the free trial period you just cancel your plan and you've had 14 free days of Shopify where you could accept payments. Am I right?