They're not open Sunday's but they do have a key drop box for returns. You can go to their website to confirm. The airport location might be open on Sunday's. Look them up and give them a call to find out. Good luck!
No and you can't return it on a Sunday either
I don't know if they do, but I believe not. I've rented there but don't work there. Google their days and hours of operation and you should get a quick response.
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