Is it possible to basically book a trip using both points and cash? For example, if one were going from CHI to ABQ (one zone) in a roomette, it's 15,000 points. If I only had 10,000 and couldn't buy anymore, could I use those 10,000 to get a discounted rate on the roomette (and pay the rest with cash/credit), or would I have to wait until I got the 15,000?