I think I have found it, let me explain it. Your cars and tariffs are very complex at the moment and anyone will be lost there :)
Booking Summary: 19-11-2023 ( Sunday ) 17:55 21.57 kilometers (Each Way) 23 mins
Killorglin, County Kerry, Ireland
Killarney, County Kerry, Ireland and RETURN: 20:11-2023 17:55 ( Monday )
Day tariff ( https://booking.drivenot.com/administrator/index.php?option=com_tbsaas&view=tariff&layout=edit&id=1570 ) is not available on Sunday,
so the system was trying any other tariff which is assigned to a car.
Let's pick a car (Skoda Superb Standard (Day) https://booking.drivenot.com/administrator/index.php?option=com_tbsaas&view=car&layout=default&id=5983 ).
Only Day tariff was assigned to this car. This is why, on 19-11-2023, no tariff was found on 19-11, but this car is set to use Tariff.
Hence, the pickup to dropoff price calculation was 0 on 19-11-2023 for this car.
But, for the return trip on 20-11-2023, Day tariff was assigned properly. ( see the attached )
So, the summary is although you have created different tariffs and cars for day and night,
but if any tariff is not available on a weekday and the car is forced to use Tariff, you have to assign a fall back tariff for that.
I have assigned Night Tariff (NT2) for the above car and now price calculation seems correct.
Please check on your side and let us know.
Thanks & Regards
Ronniee