It's not just manpower they're scrambling for this time of year. Around here, they're using Budget and Ryder rent-a-trucks for deliveries to (and pickups from) shipping stores (UPS stores, places that offer mailbox service, etc). Uniformed UPS drivers, rented box trucks.
Talking to the owners of one such place, they said UPS has done that the last few years, freeing up more of their recognizable brown trucks for residential delivery.