Advantage rates are guaranteed from the date of booking for 365 days at all
Corporate-owned locations in the US. If your reservation shows a rate is subject to change, it's recommended that you re-verify the rate prior to arrival to pick up your vehicle.
If you have a printed confirmation that states that the rate is guaranteed, I'm not sure why they would try an ignore the price. I would double check the information on your printed reservation to make sure that it doesn't state that the rate is subject to change. Since you appear to be a loyal and frequent renter, this issue is something that you shoudl discuss with the manager of the branch that your rent from.
This works the same with other car rental companies. When you book, they are required to tell you if the rate is subject to change. When reserving a guaranteed car rental rate, the price of the booking is usually only subject to change if you make changes to the original booking. That's why you often see the phrase approximate total or estimated total when you researching rates. This is because the total given is based on what you say you will do (pick up date and time, return date and time, etc)... the actual amount charged at the end of the rental will depend on what you actually do, if different that your original plans.
If you run into a problem at the rental counter with an employee refusing to honor a guaranteed car rental rate, you should ask to speak to a supervisor right away.
Pages from this site, relating to this topic:
Making Car Rental Reservations
Comparing Car Rental Rates