Recently purchased a few e-certificates through AMEX to apply $400 or to my upcoming Hawaii trip

When going through Avis's "buy it now" function, there is no place to apply this certificate. When calling, they're telling me that I cannot use this feature and must go through the regular booking, which adds about an additional $100 per rental to my trip.

Is this accurate? It totally dilutes the value of the certificate and i would not have purchased them if I had known this prior

Weak !