When you think of good middle class jobs, you probably think of UPS. But the shipping giant has a secret.

While full time delivery drivers are the face of UPS, it’s the part-time workers like Cesar Mendoza who are the heart of the $100 billion shipping behemoth. In fact part-timers make up around 60% of the 340,000 UPS workers represented by the Teamsters.

These workers have been ignored for nearly 30 years, but as UPS and the union wrestle over a new contract—with a massive strike looming July 31st—part-time workers are coming out of the shadows to tell their story of what its really like to work for the world’s largest package delivery company.