You should be okay:
http://www.hotwire.com/customer-care...-plans-faq.jsp
Q: Can I change my pick-up or drop-off time?
A: No, but you can pick up your car later than the scheduled time. Our partners will hold your rental car for up to 24 hours. If you return your car early, you will not be reimbursed for any unused portion of your reservation. If you return your car more than one hour late, you may be charged for extra rental days. Because your Hotwire Hot-RateŽ is only guaranteed for the rental period you booked on the site, any additional days may be charged at a higher rate.
But I'd still call the car company Sunday night and let them know you are "running late", and bring a copy of the Hotwire page and the Hotwire phone number (is there one?) with you, just in case.