Algo starting to choose UPS over USPS

For many of my listings, I have flat $9.99 shipping, for any amount. I find this helps have people by 2 or 3 items without killing me in shipping, but still makes it a better deal.

My listings say "Economy Shipping 2-10 days $9.99 + $0 per additional"

For years and years, eBay shipping would choose USPS. This week, I've noticed all my business is going to UPS for the same items, and it's often $2 or more cheaper. Surely someone at eBay is noticing this?