Next-gen USPS mail trucks are only capable of 8.6 mpg, EPA says
arstechnica.com
arstechnica.com
The new vehicle has greater capacity.
Potentially, this can reduce the fuel overhead for travel from the post office to the route start and from the route end back to the post office.
Also potentially, the greater capacity will reduce multiple vehicles serving the same route, e.g. one for regular mail and one or more for parcels.
The important measure is fuel consumption per mailpiece.
A switch to EV's would/will require coordination with thousands of local and regional electric service utilities to upgrade electric capacity [1] and letting of thousands of contracts to physically adapt post office truck yards. Retraining service personnel and acquisition of service equipment too.
[1]: Most likely with multiple service drops for redundancy as is typical of important facilities. Also generators for emergencies like disasters because unlike petroleum, electricity can't be stored in tanks.