Third party certification is pretty much optional but can effect sales. From a Liability standpoint you might want it.
ETL is faster than UL but sometimes UL is cheaper. There are a lot of costs associated with maintaining the certification but ETL follow-up services tends to be cheaper but more expensive for the actual test program.
UL has offices in Long Island, Chicago, RTP North Carolina, Santa Clara CA. ETL is all over but I only know teh Totowa NJ office.
If the products are in "Power Limited Circuits" such as powered by a Class 2 transformer or Class 2 power supply the risk of fire and shock is low.
BTW I worked for UL for 12 years but I am not an expert on the types of products you mentioned. I think they might fall under "TIRE" Temperature Indication and Regulation Equipment or just HVAC.