Shares of United Parcel Service suffered their worst day ever on Thursday, after the package-delivery giant announced an agreement that would cut its business with Amazon.com Inc., its largest customer,

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 CEO Carol Tome said on a call with investors that Amazon is UPS’ largest customer, but it is not the company’s most profitable customer.