Just made a reservation through Costo's travel website. It honestly was the cheapest rate for my car size I could find (tried tons of coupons, AAA, etc. with several different companies). It took my Wizard number fine with no change of rate.

But there was no input field for any airline frequent flyer number.

I tried logging onto Avis.com using my Wizard number and while I see the reservation, there still is no input field on any of the "modify" options.

Are FF#'s not allowed with Costco reservations?