In my rural area, one of the "last mile" problems is that there's now an extra leg. Since the small local post offices no longer hand-sort mail, but drive it to a larger hub post office that has the address OCR machines to auto-sort it, a cross-town letter that used to be delivered same day or next day now takes 3 days or more due to the extra round trip. Add that on top of all the extra work the USPS is doing with pandemic-related package shipment and I'm sure there is a lot of backlog at many points in the system.