I wondered how long. it would take before they did this. Businesses are good to start with because tehy are used to shipping via a carrier, Amazon is just one more carrier to see how much they can save with them vs UPS/FedEX/USPS.
Home is harder because the last miles is variable; so you can't set fixed pickup routes and handling pickups takes longer tahn delivering and also need sorting so adding it to the drop-offs would be a challenge. However, a partnership with USPS to handle last mile and Amazon ships makes sense, FedEx and UPS already handle mail so it's not some new idea; Amazon could bid for the contract, and winning deprives competitors of lower margin freight that can be used to cube or weight out a plane. Amazon, meanwhile, gets volume for deadheading planes and trucks.