shipment-tracking-custom.php

<?php   // Remove this if not required.

add_filter( 'wc_shipment_tracking_get_providers', 'custom_woocommerce_shipment_tracking_provider' );

function custom_woocommerce_shipment_tracking_provider( $provider ) {

	$provider = array(
			'United States' => array(
				'UPS'
					=> 'http://wwwapps.ups.com/WebTracking/track?track=yes&trackNums=%1$s',
				'USPS'
					=> 'https://tools.usps.com/go/TrackConfirmAction_input?qtc_tLabels1=%1$s',
			),
		);

	return $provider;
}

Thanks for this. Definitely add this to the plugin docs to help others if you haven't already :)
👍