It's only 2 per unit. It's for an automated machine, basically a few hundred cycles/day. It will be used to energize a coil with 110V AC to control an air cyliner. I'm looking at using one of IR's photovoltaics: PVA33 series -
Go with a solid state relay, this should give you good isolation with excellent reliability.
I am an automation troubleshooter and the first 2 things i look for in most any problem is (grounds for circuits, and things in the circuit that move.(ie. switches, relays with moving parts, fans etc.)