Good to know....

One of my jobs as a younger man was carpet installation, there are many types of foam ,some dense and some more "air" pockets. the lighter airy stuff will flatten out pretty quickly but it is cheaper. Its the same with cheap sofas, the cushions flatten out and you ware a seat into them. It may not be an issue depending on how often you actually drive your car. a recommendation would be to "supplement" possibly with carpet foam, Im sure we all know an installer or could go to the carpet store and see what they have, I have done this with my sofa cushions when they flatten out, I just restuff a layer or two of dense carpet pad into the cushions.
I do believe though with regard to the subject of this thread that the current cheap item is the thick seat foam for the bench seat, I believe the seat backs are a different material, more a burlap and filler type situation, so like said i think a possible situation is to do the seats with the foam that is offered, if it flattens out in say 3 years , open the seat back up and stuff come dense carpet pad into it and reclose .just to get the wrinkles out.