In the middle of Amazon’s “Prime Day” promotional sales rush, sixty warehouse workers walked out for more than three hours at the company’s delivery station in Pontiac, Michigan — bringing the facility to the brink of a total shutdown.

A delivery station is the last warehouse an Amazon package passes through before it is loaded into a truck or van en route to the customer.

This year’s Prime Day shopping bonanza on July 11 and 12 set a record for the largest sales day in Amazon’s history. The crush of Prime Day puts even more pressure on workers to keep up with conveyor belts overflowing with boxes that can weigh as much as fifty pounds.