This is what I found on eBay:

Update as of 7/13/26

📦 Ground Advantage Pricing Update

eBay Labels USPS Ground Advantage shipments under 1 lb to rural ZIP codes, Alaska, Hawaii, Puerto Rico, APO/FPO/DPO addresses follow the new USPS pricing rules.

For the continental United States, the tiered-weight structure remains in place.

https://community.ebay.com/forum/announcements-57928/topic/usps-rate-and-service-update-466150/

There IS a link in that note to a list of "rural" zip codes. It's over 17,000 entries long!