I personally rent from Hertz (due to Pres. Circle status and cushy CDP code) but I also know an Avis outlet and how the Avis system works.
When checking out, there is a fuel out code,
y/Gx. x = integer of /8th of how full the tank is, so it can either be 7/8th full or full (or less). This value is printed on the contract.
When it's checked in, the 'buy fuel= ?' field requires a similar response but it alone doesn't calculate the additional charge.
The agent needs to calculate this manually and add it in at a separate field.
I mention this just in case any sites try and say 'the system calculates a min of 1/4 of a tank $3.50/l" this is B.S. The site needs to keep the purchase receipt as the refuel gets charged to a BP fleet account, and then the litres * 3.50 is added to the booking.