They ARE trucks but UPS started calling them package cars a long time ago so they could say it;s OK for them to deliver in areas posted no trucks allowed.
They're trucks. Sounds dumb to call anything (larger than a 500) that we drive in the modern era "package cars" because they're not cars. They're long, packed with hundreds of pieces and walked front to back for 8-14 hours daily. Anybody who insists on calling them "cars" is just trying to tow the official company line. Hearing somebody in management say "No, they're package CARS" makes me cringe.