Lost session variable - should I be doing something different?

[Supertex]

So the fedex web services module that's available in the addons section will pull quotes, but most of the other features don't work...specifically insurance.  Soooo...I tried to a cart value check and add the appropriate fees to the shipping method from within the module.  I couldn't get that to work.  But what I could (and did) do was calculate the appropriate fees in shopping_cart.php, and put that into a session variable:

//set insurance figures for fedex shipping methods	
	$pkg_val = $cart->show_total();
	if ($pkg_val <= 100){
		$insurancemarkup = 0;
	}else if (($pkg_val > 100)&&($pkg_val <= 300)){
		$insurancemarkup = 3;
	}else if ($pkg_val > 300){
		$insurancemarkup = (ceil($pkg_val/100));
	}
	tep_session_register ('insurancemarkup');

Then I did this in the fedexwebservices.php file itself:

 function quote($method = '') {
    /* FedEx integration starts */
    global $shipping_weight, $shipping_num_boxes, $cart, $order, $insurancemarkup;

And and then this also:

          $methods[] = array('id' => str_replace('_', '', $rateReply->ServiceType),                                                   
                             'title' => ucwords(strtolower(str_replace(array('_', ' HOME', 'DELIVERY'), array(' ', '', ''), $rateReply->ServiceType))) . '  <font color="orange">(signature may be required)</font>',
                             'cost' => $insurancemarkup + $cost + (strpos($this->types[$rateReply->ServiceType]['handling_fee'], '%') ? ($cost * (float)$this->types[$rateReply->ServiceType]['handling_fee'] / 100) : (float)$this->types[$rateReply->ServiceType]['handling_fee']));

This works fine...most of the time.  However, someone will occasionally manage to place an order and have $insurancemarkup as zero. Handling fees are correctly calculated, prices are coming back correctly...it just doesn't add the insurance.  I don't know if they're creating a cart from work and then coming home to actually place the order, or...?  Again...this is a rare occurrence but it can really cause problems; especially when I cannot replicate the error.  Is there a way to successfully make the calculations from within the module?  Have I gone about this in a foolish manner?

Thoughts?

[Supertex]

@kymation