More than 80 contract drivers and dispatchers who handle deliveries for Amazon .com Inc. have joined the International Brotherhood of Teamsters.

Amazon trusts customers to provide an honest rating of their delivery driver. But what if the customer is biased against the driver?

Amazon.com’s results were marred by a surge in costs tied to its one-day delivery push, worrying investors who had grown accustomed to almost two straight years of fattening profits.

Amazon.com will spend $800 million in the current quarter to increase delivery speed for Prime members to one day from two, the company announced April 25.

Amazon.com will use robots to deliver packages in the suburbs north of Seattle, its latest experiment to automate the last mile of delivery that is a labor-intensive and costly component of buying products online.