Hello ZenCart Community!

I've recently installed Zencart v1.3.9d with no addons.

I am attempting to integrate a 3rd party affiliate program called Affiliate Trail to my shop. The integration seems simple enough, I just add this code to my confirmation page:

<img src="http://www.yourdomain.com/affiliate/scripts/sale.php?TotalCost=
XXXXXX.XX&OrderID=XXXXXX&ProductID=XXXXXX" width=1 height=1>

Where src="the page on my domain"
Where TotalCost= the orders total cost

OrderID and ProductID aren't neccesary so the new code looks like:

<img src="http://www.yourdomain.com/affiliate/scripts/sale.php?TotalCost= XXXXXX.XX" width=1 height=1>

My question is therefore:

What is the variable that Zencart uses as a "TotalCost" once an order is confirmed?