Please advise on mains relay type
There are latching relays that are turned on by a momentary application of power to one terminal and turned off by a momentary application of power to another terminal. If the application of power to either terminal is continuous, you could design a relay circuit that would disconnect the power automatically. There may be latching relays with that built in. You need to search to see what is available. You should be able to find products that are described in sufficient detail to allow the customer to understand the function. If a product is not well described, look elsewhere.