No codes. Just go through costcotravel.com It applies any and all coupons out there for you and you still get the pre check in and skip the counter options. I've done it this way many many times.
Side note, I've crossed searched any coupon and contract codes I can find and consistenly find Costco to be the best.
Another thing I do is play with the times of pick up and drop off. Some times being 6 days and 23.5hours is cheaper by a lot then 7 days.
I also check every day! ( I trip plan for a lot of friends - (I'm not a
travel agent of any kind) but help friends so I'm constantly checking for all their trip and mine to Disney and other destinations and Costco kills it every time!))