How long is it until your rental start date?

Often at Avis (which is co-owned with Budget), I find that rental prices go up, and up, and then often suddenly sometime between two weeks out and a few days out they drop for a short time to lower than they ever were before. In that case, I always Modify my reservation to take advantage of the new lower rate.

So what you may want to do is monitor often starting at two weeks out to see if the price has actually dropped, and then if it has, call right then to see if you can get your Fastbreak # added and get the latest (now lower) price at the same time!

Or, if you prefer, assuming your reservation is cancellable (with no card charged yet), once you see a lower price, book the new reservation with the Fastbreak# at the lower price, and cancel the original reservation. That you can probably do online without any calls.