I don't think the urban areas would work/are needed. Right now all mail is delivered by foot and likely faster than you could deliver with any vehicle, lots are 25-30feet wide so the density makes walking the best method. Where I do see it happening is out in the suburban subdivisions where people don't even have personal mailboxes, there's usually a cluster of boxes in the middle of the block for all the residents. So deliveries would be every 400m or so, electric would be perfect.