One idea would be to pay customers to deliver packages. Then you can incentivize them to come in to buy something and then take and deliver some packages for you.
It'd be much harder to keep people accountable. Who do you blame when the product arrives damaged (or not at all)?
There's probably some way to incentivize good behavior. I would do it for my neighbors within reason. That being said Amazon is still leading the way, maybe Walmart can do USPS for the last mile as well.