Chances are, you'll find a cheaper weekend deal than if you pay one or two days' regular rate and get one free day with the certs (plus that way you'll get some miles for the rental), but you should check with all of the rental cos. to be sure. If you're not running up against an expiration date, maybe you should hold on to the certs until you only need a one day rental or have claimed a couple more awards and have some more certs. If you do combine them, you tell the rental co when you make the reservation and generally just turn them in when you pick up the car.