Shares of Amazon.com Inc. AMZN, -3.95% slumped 3.8% toward a 3 1/2-month low in afternoon trading Friday, after BofA Securities analyst Justin Post said he believes sales from the ecommerce giant's recent Prime Early Access event were less than the Prime Day event in July. The stock extended the 0.3% decline on Thursday, which came despite the S&P 500's SPX, -1.91% 2.6% surge on the same day.