1) you assume that the Amazon rates are sufficient to cover variable costs,
2) long term, the Postal service’s fixed plant should be sized to handle profitable business, not business which can only be generated at a loss.
With large, bureaucratic organizations like USPS that are burdened with union rules that make for highly inefficient operations, more and more of their labor costs are fixed, not variable. The irony is that this "institutional inefficiency" makes it much EASIER for a third party like Amazon to come in and pay very little for the USPS services and still cover its variable costs.