I'm a little confused about how you would like this to work. From the perspective of the user they will be logged into your site and choose items to add to the cart. When they get to the checkout page they would enter their information there. You would want to store this information along with the order details.
From this point I'm not sure how you would like it to work. How are you collecting the cash from them? It is when you collect the cash that you should update the order to reflect that payment has been made.