On the Budget site, there is a link at the top for "specials." On that next page, about 2/3rds of the way down on the right side, you will see "Mastercard Promotion" That will bring you to a page that offers "Save up to 20% on a rental when you use your MasterCard® . Plus, get an additional 1-car class upgrade!" Don't forget the coupon, because you never know how long it will be out there.
You didn't say when you needed the rental, but this is good through the end of the year. (and it assumes you will be using a Mastercard, of course.)
Funny thing is - there is no place to enter the coupon, just the discount code. I called the help line, and she said that using the code alerted the agent that I would be asking for the upgrade. You may want to check that out, and post what they tell you.
I got the very best rental rate from that promotion, in addition to the upgrade.
Maddle