Since it was a parking lot incident, fault can be difficult to determine. If you two aren't able to come to an agreement on the repairs, your best bet would be to just contact your insurance company and let them know about the what happened. This will remove you from having to negotiate with the other driver about the repairs. Given the fact that your vehicle was in reverse at the time of the crash, you might actually be considered at fault even if the other driver was speeding. The determination of fault will ultimately be up to your and the other driver's insurance company.