Found the answer digging around at budget.com
When I enter a coupon code, why doesn't budget.com reflect my discount in the final price?
Like using a coupon at grocery store, if you have a coupon for dollars off or for free rental days, we’ll apply the discount when you return and pay for your rental. If your coupon is for an upgrade to a bigger car, your rental location will have to confirm availability of that vehicle once you arrive at the rental counter. Either way, be sure that you use a coupon or discount code that meets all terms and conditions of the offer before the deal expires. Coupon codes are always four letters followed by three numbers – for example, LLLL###. A Budget Customer Discount (BCD) Coupon is always one letter followed by six numbers – for example, L######.