The voltage specification of the UL certified product is 115V, but can it not be used in areas of the United States where the power supply voltage is 120V?

Yes, you can. Testing for UL certification is performed at 120V. The American national standard ANSI C84.1 stipulates that motors and their control devices used in an environment with a nominal system voltage of 120V shall have a rated voltage of 115V ±10%. Similarly, motors and their control devices used in an environment with a nominal system voltage of 240V shall have a rated voltage of 230V ±10%.