Okay, let me summerise this, and make changes as you see fit.
1, You're Avis First and wanted a compimentary upgrade for your rental.
2, You were upset that they wouldn't upgrade you over the phone and had asked you to go inside to request an upgrade and recieve your paperwork for the new car.
3, You chose to take a vehicle that was not issued to you assuming that you could just take your upgrade that you felt was yours.
4, when you got the new paperwork you chose to not look at the newly issued paperwork to review what charges you'd be billed for and then left.
5, when returning the vehicle you chose to not look at the amounts that you were billed for on the paper receipt.
6, when you finally got around to checking the amount due you were upset that a few weeks later it was larger than you anticipated, when this could have been resolved BEFORE leaving the airport.
7, you're disputing this with Amex (which will likely not work)
TL;DR - I was issued on rental car, and then took a different one with out asking about the cost and am now upset that it is $190 for the day.
Being Avis First (and Pres Plus to boot) myself, I've been upgraded pretty regularly, but not all the time. In the "grab and go" cars if I hadn't liked the car that I've been issued going inside is not that much of a hassle and I've generally been able to get what I want.