different city but same zip is fairly common. what happens is townships sometimes share a po, and while the address for the house might be one city, the zip is another one. (if that makes sense)
luckily i've never been in this situation. the closest to this ive had was one time a buyer apparently had the delivery person hold it for a bit before delivering, cause he filed a not-received, and once he got a replacement, the next day he told me that the original one arrived and that he had refused delivery on it. then the next day i get an email from another person asking for lowest i'd take for the same exact ring, in coicidentally the same size.
since at one point the customer told me he was friends with his delivery person, i think he and friend were trying to pull a fast one and get me desperate to sell an item and sell it cheap.
I've still got the extra ring, and no way am i going to offer it online cheap. i'll sell it at a fair for cost before i do that. (state fairs is our primary venue).
back to your prob. have you provided the person with the fact that DC shows it delivered? if not, i would tell them that it shows delivered, and suggest that either a family member knows where it is or that maybe it got delivered to neighbor and to check with neighbors. and if all else fails, to ask postal delivery person if s/he remembers delivering it.
hopefully it'll magically appear.
i'm about to call and fuss at post office here myself, parcel i waited and waited on, come to find out they delivered to another house in neighborhood. which, it's a small neighborhood, so i got it. but took an extra week to get here cause has been sitting on their porch all this time while they were out of town meanwhile, the rebates for the merchandise inside have expired. if i cant get them to honor the rebates, even though late, will be out $100. and you'd think that the po of all people would know that around here the even numbers are on the south side of road. but anyway...
best of luck with your situation! hope it gets resolved. if doesnt get found, hopefully insurance will pay up even though DC shows it was delivered. (or will show up when the buyer finds out has to write sworn statement that it got lost and that falsifying it is insurance fraud and a felony)