Last week, two well-established Amazon delivery companies in Portland, Oregon offered Amazon an ultimatum: agree to a set of conditions that it said would improve revenue and driver safety—or we'll stop delivering Amazon packages.

Amazon refused, and the two companies in the Portland area terminated their contract with Amazon, their only client, effectively shutting down.

"Amazon's conduct over the past two years has become intolerable, unconscionable, unsafe, and most importantly, unlawful," a letter sent to Amazon by the attorney of the two delivery companies, Triton and Last Mile and obtained by Motherboard, reads.

The incident is notable as it appears to be the first public example in the United States of Amazon delivery service partners, small businesses that deliver packages exclusively for Amazon, using their leverage to protest against Amazon—which has been known to enforce strict rules that squeeze productivity out of their delivery drivers, putting drivers and the public at risk.