I'm looking for a plugin that would pull in USPS rates using Stamps.com for version 1.9 (aka not Magento 2). Does such a thing exist?
There is the combination of apps that helps handle USPS shipping with Stamps account shipping rates - Store Manager for Magento + addon to is called USPS Shipping Integration. It allows to create shipping labels on the basis of Stamps shipping rates -
You can check more details and download free 2-weeks trial version at: https://www.magentocommerce.com/magento-connect/shipping-integration.html