Just change drop off to 8AM, Wwhen the website show cars it is possible. I just checked for you, it is possible:-URL: [link]
Client
OK, did you change it?
Agent
Did you make the booking already?
Client
Yes, I just booked.
Agent
Ah OK. I see you booking comin in now. Well you can return the car at anytime...if they wouldn't be open, you are able to use the key drop box facility but they are open at that time.
Client
OK, I was just worried I wouldn't be able to drop off the vehicle. Thank you.