This was my response from Budget. They don't seem to understand I'm not looking to "modify", all I want to do is link my Fastbreak account to the reservation!
"Rate structures are similar to airline pricing, our prices change depending on car availability, location popularity, and many other factors. This is why prices can fluctuate on a minute to minute basis. Unless it is an advertised special offer, there generally is not one set rate for a particular type of car, but rather a range of prices depending on the length and dates of your reservation. Even with special offers, the advertised rate usually applies to a limited supply of cars which varies daily by make and model at each location.
Please note we are unable to modify rates via e-mail for this reason."