USPS should be integrated as a shipping carrier via their own APIs. There's one for the standard postage using USPS web tools. There's also a business customer gateway and they might have an API for business rates as well. Both should be implemented as carriers as most USA sellers ship USPS since they're the cheapest on average. Endicia costs money to use their service and not everyone has that service or wants it, so those without it cannot set up their USPS shipping method.