What should I do if I encounter an issue with the parcel locker key on my USPS route?

When I first took over a USPS route with many apartment buildings, I encountered an easily underestimated problem: the master key for the parcel lockers was temporarily missing, while the scanner already showed multiple packages as 'out for delivery' and residents were waiting for notifications. It wasn't the kind of failure where the cause is immediately obvious; it was more of a combination of…