Just my opinion: I would add it myself. Not worth the hassle of trying to get Motability or the hire company involved.
You can also buy it for far less than £14 for 5 litres. Just check out the discount retailers like Home Bargains, B&M Bargains etc, think I’ve seen it at around £8.50.
2024 - BMW i4 Grand Coupe eDrive 35 Sport
2020 - Volvo XC40 T4 Inscription
2017 - Audi Q3 TFSi Sport S-Tronic