That is exactly what happened to me, except in my case the surcharge was over $600. I knew I had been upgraded but like others here I signed on the electronic screen not realizing I would be charged for it. This was at FLL Fort Lauderdale airport (so the USA again) and the problem was compounded by the fact that I returned the car to MIA airport and they could do nothing about it. So I for one will avoid one way car rentals in the future.
It took quite a lot of phoning and emailing once back in Europe to sort it out and I got a good chunk of it refunded but not all. I took that as a lesson to be more careful in the future. But I feel it was a very dishonest and underhand practice by Avis. Remember, agents look for commissions from upgrades.