United Parcel Service Inc. shares plunged after the company projected annual revenue well below expectations, telling investors that a long-awaited rebound in demand for its parcel services won’t arrive this year and prompting it to slash its low-margin business with Amazon.

UPS is breaking away from Amazon.com. The package-delivery company said it was cutting volumes from its largest customer by half, and warned its revenue would come in about $6 billion lower than Wall Street expected this year.