1.6.x Payment Module - Failing Validation

[cjrowell1980]

Im trying to create a payment module but stuck at the validation step

after visiting a third parties site to process payment, it returns to my validation script but always fails

the posted values from the 3rd party site are (order_number, transaction_status, transaction_value, date_time, authorisation_code)

I try to recall the cart using

$cart = new Cart((int)Tools::getValue('order_number'));

then check if valid

if ($cart->id_customer == 0) {
    $this->returnError('Invalid Cart', $postValues); // send email on fail of posted values, then kill script
    }

I get an email informing of a failed validation

I suspect im missing something super simple, but cant see it (ive included my file for ref)

<?php

class payacardValidationModuleFrontController extends ModuleFrontController {

    public function postProcess() {

        // kill process if module is not active
        if ($this->module->active == false) {
            die;
        }

        foreach ($_REQUEST as $key => $value) {
            $postValues .= "{$key}={$value}";
        }

        // restore cart context
        $cart = new Cart(Tools::getValue('order_number'));

        if ($cart->id_customer == 0) {
  //    if ($cart->id_customer == 0 || $cart->id_address_delivery == 0 || $cart->id_address_invoice == 0 || !$this->module->active) {
            $this->returnError('Invalid Cart', $postValues);
        }
    

        // set datas
        $transaction_status = Tools::getValue('transaction_status');
        $transaction_value_pence = Tools::getValue('transaction_value_pence');
        $date_time = Tools::getValue('date_time');
        $authorisation_code = Tools::getValue('authorisation_code');
        $paya_signature = Tools::getValue('paya_signature');
        
        // check payacard signature
        $tokenPreHash = $cart->id.':'.((float)$cart->getOrderTotal(true, Cart::BOTH)*100).':'.Configuration::get('PAYACARD_ID').':'.Configuration::get('PAYACARD_SIG');
        $tokenHash = hash('sha512', $tokenPreHash);
        if ($paya_siganture != $tokenHash)
            $this->returnError('Invalid Token',$postValues);

        // validate order
        $currency = new Currency((int)$cart->id_cuurency);
        $total_paid = $transaction_value_pence/100;
        $extra_vars = array(
            'transaction_id' => $authorisation_code,
        );

        if ($transaction_status == "A" && $transaction_status != "D") {
            $this->module->validateOrder($cart->id,
            Configuration::get('PS_OS_PAYMENT'), $total_paid,
            $this->module->displayName, NULL, $extra_vars,
            (int)$currency->id, false, $customer->secure_key);
        } else {
            $this->returnError('Payment Declined',$postValues);
        }

    }

    public function returnError($subject, $message) {
        Mail::Send(
            (int)(Configuration::get('PS_LANG_DEFAULT')), // defaut language id
            'contact', // email template file to be use
            'Payacard - Failed Validation - '.$subject, // email subject
            array(
                '{email}' => Configuration::get('PS_SHOP_EMAIL'), // sender email address
                '{message}' => $message, // email content
            ),
            Configuration::get('PS_SHOP_EMAIL'), // receiver email address 
            NULL, //receiver name
            NULL, //from email address
            NULL  //from name
        );
        exit;
    }
}