The USPS has recently changed its service packages of First Class, Parcel Select, and Retail Ground services. These services will now be offered under a new service called Ground Advantage. I noticed that the Ship Via choices for stamps.com and EasyPost do not support these recent changes.
I have not seen any information from USPS about when they will be retiring these services, but I found a related article from EasyPost.
https://www.easypost.com/blog/2023-06-06-upcoming-usps-price-and-service-changes-in-july-2023
Users will be able to buy USPS Ground Advantage shipments using the new "GroundAdvantage" service name or the legacy "First" or "ParcelSelect" service names through Jan 21, 2024, at which time the legacy service names will no longer be supported.
https://faq.usps.com/s/article/USPS-Ground-Advantage