wps-shipping-options-reordering.php

<?php
/**
 * Change shipping method priority
 * Use array_unshift for the beginnig and array_push onto the end of array
 */
function kmchild_sort_shipping_methods($available_shipping_methods, $package) {
	if (isset($available_shipping_methods['wc_pickup_store'])) {
		$wc_pickup_store = $available_shipping_methods['wc_pickup_store'];
		unset($available_shipping_methods['wc_pickup_store']);
		// array_unshift($available_shipping_methods, $wc_pickup_store); // Prepend element to the beginning of array
		array_push($available_shipping_methods, $wc_pickup_store); // Push element onto the end of array
	}

	return $available_shipping_methods;
}
add_filter('woocommerce_package_rates', 'kmchild_sort_shipping_methods', 10, 2);