Who should be contacted for a package left in the rain?

A customer of mine has requested a refund due to the USPS delivery person leaving their package in the rain instead of under the porch. I put a declared value when purchasing shipping. Should I be held responsible and accept the return, or should the customer attempt to get a refund through contacting USPS as it was their fault for leaving it in the rain when there was a covered section to leave the package under, thanks!