Changing the default shipment provider - https://docs.woocommerce.com/document/shipment-tracking/#section-5

change_default_provider.php

add_filter( 'woocommerce_shipment_tracking_default_provider', 'custom_woocommerce_shipment_tracking_default_provider' );

function custom_woocommerce_shipment_tracking_default_provider( $provider ) {
	$provider = 'USPS'; 
	return $provider;
}