Shop Till You Drop... Dead by R.L. Stine

You're sick of Reggie Mayfield's tall tales. Especially the one about his dad's department store. Reggie says monsters roam the store late at night. You bet Reggie is lying. Too bad for you, Reggie is telling the truth! Monsters do roam the store. Like the Vampire night guard and the mannequin that comes to life. Watch out, he's dressed to kill! Can you survive the midnight hour at Mayfield's?