I don't rent from Budget for the same reason. Every transaction is a new damn chess match / battle of wits with people who are trying to hurt you.

When we have our periodic fuel price jumps in the US there is a run on compact cars in the rental lot and they sell out. SUVs and V6/V8 cars gather cobwebs. The agency has to get late-arriving customers who reserved compacts to accept bigger vehicles.

The first tactic is to try to sell the upgrade, although (A) they'll eventually have to give it to you anyway because it's all they have left, and (B) they should be paying you, not vice versa, because you'll be spending much more on gas than you expected.

Second tactic is to award you the upgrade like it's a prize instead of a burden, but then charge sneakily for it without saying so, which is what happened to the OP.

Only after you refuse both those deals and scrutinize your rental contract like Jerry Maguire do you have a chance of fair treatment.

In the whole travel world, car rentals are the clearest case of "you get what you pay for." I stick to Avis and Hertz. No trust in any of the others.