I'll guess you did something in business policies to change across the board. Also, your listings are showing a more expensive Fedex ($19 to me) vs an 'optional' (which no buyer will see or know how to choose) UPS at $14.
Anyway:
go to your 'my ebay' (seller hub or whatever first page takes you to)
top left hover over your name
at drop down click 'account settings'
scroll down, find 'business policies'
Find the Fedex/UPS and either change it or create a new one for USPS and then 'zero out' or delete the 'old one' (fedex/ups)
