add_action('processed_subscription_payment', 'custom_process_order', 10, 1);

function custom_process_order($user_id) {

    $amount = 9;
    $funds = get_user_meta( $user_id, 'account_funds', true );
    $funds = $funds ? $funds : 0;
    $funds += floatval( $amount );
    update_user_meta( $user_id, 'account_funds', $funds );

}