wcpbc_country_selector widget should return, not echo

ob_start(); // add this

wc_get_template('country-selector.php', array( 'countries' => $countries ), 'woocommerce-product-price-based-on-countries/',  WCPBC()->plugin_path()  . '/templates/' );

return ob_get_clean(); // and this

https://wordpress.org/plugins/woocommerce-product-price-based-on-countries/