Aluminum is only ~2,300$ per metric ton, so the raw material cost is not that much of an issue. It’s material property’s is actually a significant issue as the cars vibration for example would likely cause cracks to form and propagate.

PS: Ford is moving the F150 to aluminum adding ~500$ per vehicle but also saving weight. Which should net to zero via added fuel economy.

For something on the scale of the USPS fleet, economies of scale would come into play. If such vehicles were very highly modular, all of the modules could be removed and placed into another frame and the old one recycled. (Come to think of it, this is also somewhat done with Abrams M1 tanks.)