Por fin lo encontré!!

gracias a esta web: http://www.pootlepress.com/
y funciona! además tiene muchos tutoriales sobre woocommerce

hay que copiar en el archivo functions.php del tema que estés usando, lo siguiente:

/**
* woocommerce_package_rates is a 2.1+ hook
*/
add_filter( ‘woocommerce_package_rates’, ‘hide_shipping_when_free_is_available’, 10, 2 );

/**
* Hide shipping rates when free shipping is available
*
* @param array $rates Array of rates found for the package
* @param array $package The package array/object being shipped
* @return array of modified rates
*/
function hide_shipping_when_free_is_available( $rates, $package ) {

// Only modify rates if free_shipping is present
if ( isset( $rates[‘free_shipping’] ) ) {

// To unset a single rate/method, do the following. This example unsets flat_rate shipping
unset( $rates[‘flat_rate’] );

// To unset all methods except for free_shipping, do the following
$free_shipping = $rates[‘free_shipping’];
$rates = array();
$rates[‘free_shipping’] = $free_shipping;
}

return $rates;
}