I am not sure if this is idiomatic.
The employee pumped petrol into the car and told the driver, who is a regular to the station, "there is free refill on that".
Is it ok to say "on that"? The "on" doesn't sound very idiomatic.
The use of "on" seems perfectly natural. We can use "on" to mean "there is a special offer associated with that purchase".
There's a 20% discount on men's clothing today!
The idea of an employee (I assume employed by the gas station) filling your car seems very odd to me (though I've seen it in old movies). And I've never heard of a gas station offering "free refills".