That’s a very good price for that switch. And coopers are generally very well-made.
Now comes the question of accessory switches. Accessory switches, Are also called auxiliary switches or sometimes dummy switches. They are intended to be used in a three-way set up with a master switch of the same brand and model line. The accessories switch typically acts as a “remote” to the master switch – – it just tells the master to turn the power to the light fixture on or off.
Different companies use different technology for their accessory switches. GE, for example, does not put radios of any kind in their accessory switches. They use physical traveler wires to connect them to the master. The switches cannot be used without a master and will not be recognized by smartthings because they don’t have a radio.
The Coopers are very different. The auxiliary switch does have a radio. It is a certified Z wave device and does pair directly to SmartThings. Although you can actually use it with physical traveler wires (which allows it to work even if the home automation network is down), it is more typically used for a “virtual three-way” where it sends a signal to the hub and the hub send a signal to the master.
However, and this is the really important part, the 9542 Cooper auxiliary switch does not control a load to a light fixture. It’s not intended to. The expectation is still that the master will control the actual current load. So even though this is not a battery operated switch and it does get its power from the mains, you can’t use it to control a light fixture directly. You have to have a master to do that.
Now for the really fun part! There are people who use the 9542 by itself to control zigbee smart bulbs. Or anything else that doesn’t require actually controlling the current load. So you do see people buying them without buying a master. They can pair to SmartThings on their own, and they can be useful devices in a SmartThings set up. But they cannot control current load.
The following thread discusses how people use the 9542 with smart bulbs. 
