Initiates the Checkout process by checking out the user's shopping cart.

If the Payment Processor associated with this Order requires a Card Reader, it will generate a Card
Reader Request and send a success message with an 'orderStatus' of InProcess. This
essentially means that the implementor needs to call the Card Reader with the provided request, perform the transaction
out-of-band and then pass the Card Reader's response to the API on the 'finalize' URL provided in this response to
settle the Order.