This situation is common and unfortunate. It's frustrating to be charged more for less days.
As a reservation agent, I was taught that the rental is charged based on what you actually do.... not on what you say you will do, and with the companies I worked for, there were always minimum and maximum requirements for things like weekly rates and monthly car rental rates. Read more about Comparing Rate Quotes
Any monthly rate for car rental that I've seen does require that you keep the car for at least 28 days.
All of the companies that I'm aware of, use computerized systems for calculating both quotes and rates... and if a vehicle was booked for 28 days, but returned earlier, the computer would automatically adjust the rate to a weekly + days because the minimum requirement had not been met for the monthly rate. I don't think they look at it as a punishment, it's just that when they set terms and conditions for rates and rentals, there is rarely any leeway or wiggle room... returning a car a minute late over a given grace period can result in a full days charge.
I would guess that only a manager would be able to authorize an override of the rate regulations, but I doubt that you would be able to find one to do so.
The only thing that I could suggest is to check and see if it is possible to get the same monthly rate for car rental on a 31 day rental. If so, and you could book for 31 days, and pick it up 3 days early and then return it on either the 28th, 30th or 31st day. And remember that car rental companies charge based on a 24 hour day... even if you picked it up on Day 1 at 10am and were able to keep it until after 1pm on day 27, those extra 3 hours would put you into a 28 day rental, and hopefully able to keep the monthly rate (assuming it has a 28 day minimum and a 31 day maximum).
Pages from this site, relating to this topic:
Long Term Car Rentals
Tips to Save Money on Month to Month Rentals