WP nonce verification

I’m busy testing out the plugin through Travis and the only issues remaining are:

Processing form data without nonce verification

It seems to be relating to this action:

/**
* An action that handles the call from PayFast to tell Give the order was Completed
*/

function payfast_ipn() {
$give_options = give_get_settings();

if ( isset( $_REQUEST['m_payment_id'] ) ) {

    if ( give_is_test_mode() ) {
        $pf_host = 'https://sandbox.payfast.co.za/eng/query/validate';
        give_insert_payment_note( $_REQUEST['m_payment_id'], 'ITN callback has been triggered.' );
    } else {
        $pf_host = 'https://www.payfast.co.za/eng/query/validate';
    }

    $pf_error         = false;
    $pf_param_string  = '';
    $validate_string  = '';

    if ( ! $pf_error ) {
        // Strip any slashes in data.
        foreach wp_verify_nonce( $_POST as $key => $val ) {
            $_POST[ $key ] = stripslashes( $val );
        }
        foreach ( $_POST as $key => $val ) {
            if ( 'signature' != $key ) {
                $pf_param_string .= $key . '=' . urlencode( $val ) . '&';
            }
        }
        $pf_param_string = substr( $pf_param_string, 0, - 1 );
        $validate_string = $pf_param_string;
        if ( isset( $give_options['payfast_pass_phrase'] ) ) {
            $pass_phrase = trim( $give_options['payfast_pass_phrase'] );
            if ( ! empty( $pass_phrase ) ) {
                $pf_param_string .= '&pass_phrase=' . urlencode( $pass_phrase );
            }
        }
    }
    $signature = md5( $pf_param_string );

    if ( give_is_test_mode() ) {
        // translators:
        give_insert_payment_note( $_REQUEST['m_payment_id'], sprintf( __( 'Signature Returned %1$s. Generated Signature %2$s.', 'payfast_give' ), $_POST['signature'], $signature ) );
    }

    if ( $signature != $_POST['signature'] ) {
        $pf_error = 'SIGNATURE';
        $error   = array(
            'oursig' => $signature,
            'vars'   => $_POST,
        );
    }

    if ( ! $pf_error ) {
        $valid_hosts = array(
            'www.payfast.co.za',
            'sandbox.payfast.co.za',
            'w1w.payfast.co.za',
            'w2w.payfast.co.za',
        );

        $valid_ips  = array();
        $sender_ip = payfast_get_realip();
        foreach ( $valid_hosts as $pf_hostname ) {
            $ips = gethostbynamel( $pf_hostname );

            if ( false !== $ips ) {
                $valid_ips = array_merge( $valid_ips, $ips );
            }
        }

        $valid_ips = array_unique( $valid_ips );

        if ( ! in_array( $sender_ip, $valid_ips ) ) {
            $pf_error = array(
                'FROM'  => $sender_ip,
                'VALID' => $valid_ips,
            );
        }
    }

    /*
    * If it fails for any reason, add that to the order.
    */
    if ( false !== $pf_error ) {
        // translators:
        give_insert_payment_note( $_POST['m_payment_id'], sprintf( __( 'Payment Failed. The error is %s.', 'payfast_give' ), print_r( $pf_error, true ) ) );
    } else {

        $response = wp_remote_post(
            $pf_host, array(
                'method'      => 'POST',
                'timeout'     => 60,
                'redirection' => 5,
                'httpversion' => '1.0',
                'blocking'    => true,
                'headers'     => array(),
                'body'        => $validate_string,
                'cookies'     => array(),
            )
        );

        if ( give_is_test_mode() ) {
            give_insert_payment_note(
                $_POST['m_payment_id'], sprintf(
                    // translators:
                    __( 'PayFast ITN Params - %1$s %2$s.', 'payfast_give' ), $pf_host, print_r(
                        array(
                            'method'      => 'POST',
                            'timeout'     => 60,
                            'redirection' => 5,
                            'httpversion' => '1.0',
                            'blocking'    => true,
                            'headers'     => array(),
                            'body'        => $validate_string,
                            'cookies'     => array(),
                        ), true
                    )
                )
            );
            // translators:
            give_insert_payment_note( $_POST['m_payment_id'], sprintf( __( 'PayFast ITN Response. %s.', 'payfast_give' ), print_r( $response['body'], true ) ) );
        }

        if ( ! is_wp_error( $response ) && ( $response['response']['code'] >= 200 || $response['response']['code'] < 300 ) ) {
            $res = $response['body'];
            if ( false == $res ) {
                $pf_error = $response;

            }
        }
    }

    if ( ! $pf_error ) {
        $lines = explode( "n", $res );
    }

    if ( ! $pf_error ) {
        $result = trim( $lines[0] );

        if ( strcmp( $result, 'VALID' ) === 0 ) {
            if ( 'COMPLETE' == $_POST['payment_status'] ) {

                if ( ! empty( $_POST['custom_str2'] ) ) {
                    $subscription = new Give_Subscription( $_POST['custom_str2'], true );
                    // Retrieve pending subscription from database and update it's status to active and set proper profile ID.
                    $subscription->update(
                        array(
                            'profile_id' => $_POST['token'],
                            'status'     => 'active',
                        )
                    );
                }
                give_set_payment_transaction_id( $_POST['m_payment_id'], $_POST['pf_payment_id'] );
                // translators:
                give_insert_payment_note( $_POST['m_payment_id'], sprintf( __( 'PayFast Payment Completed. The Transaction Id is %s.', 'payfast_give' ), $_POST['pf_payment_id'] ) );
                give_update_payment_status( $_POST['m_payment_id'], 'publish' );

            } else {
                // translators:
                give_insert_payment_note( $_POST['m_payment_id'], sprintf( __( 'PayFast Payment Failed. The Response is %s.', 'payfast_give' ), print_r( $response['body'], true ) ) );
            }
        }
    }
}
}
add_action( 'wp_head', 'payfast_ipn' );

How would I go about to add the WP Nonce? Finding it tricky to apply.