I think the big question is whether the package density would be enough to support bikes.

Say for a city of 100,000. If every citizen orders on average one item from Amazon a year, that is 100,000 packages a year to deliver. Say at 300 working days, that is 333 packages a day (likely weighted towards holidays).

Or, 41 packages an hour for an 8 hour shift.

Likely the Post Office can hit close to 1 mailbox every 2 minutes, but that would really push it for almost any vehicle delivery, but again, it depends on how close they are, or people receiving multiple packages (different origins?).