My worst car rental experience out of 3 times (two times with other platform)

Advantage did not have the car we want. They say that their cars are for first come first serve (base on customer that come in), even though I reserved it online. We have to upgrade and pay more for an SUV. Basically they gave us an option which isn't much option to choose from. They say reserving online only lock the price, but doesn't lock the car, which then I don't understand what is the purpose of me spending my time choosing the car for so long, because in the end, the car availability is in correct and it's totally up to what advantage have.