Rather more ominous than the possible loss of money from fuel left in the car is that fact that you could be charged for damage to the vehicle, real or invented and you would be hard pressed tp prove that the vehicle was undamaged when you returned it because it wasnt inspected in your presence.
For a while I worked part time for Hertz in the UK. We would always accompany the hirer when he accepted the car at the beginning of the hire; we would point out any known damage or scratches and invite the client to do his own walk round inspection. Anything we may have missed and was picked up by the client would be duly noted on the Acceptance Sheet and signed by the client. The same damage would be reported on the Vehicle History Record Card. Upon return, whenever possible, the same rep would carry out a Return inspection. Any damage or scratches not recorded on the original Acceptance Sheet would be charged to the client.
In short use the same principle whenever you hire a car, insist that any damage is recorded on your Accepatnce Inspection sheet and always insist on a rep being present and inspecting the vehicle when you return it.
Out of hours Vehicle key drop boxes are an open invitation to less scrupulous hire companies to 'invent' new damage...
The profit margins operated by Car Hire companies are surprisingly low, especially if the company retires its fleet after just two years of operation, taking into account depreciation and wear and tear. These profits are squeezed even further by the demands of third party agencies such as 'DoyouSpain' and 'Holiday Autos' and the ever increasing costs of Road, Public and Personal liability Insurance. TBH it came as no great surprìse when the hire companies started adding fuel restrictions or additional insurance.
From a purely personal standpoint I would never use a third party agency; the agreements they have in place with the hire companies, like as not, are very different from any agreement they have with you...