Retrieving order value and other variables on reciept page? help!

Hey all, here is exactly what I need to do, I hope you can help.

I'm installing a performance tracker. This code will show up on the reciept page.

<script language="javascript"> <!-- /* Performance Tracking Data */ var mid = '*****'; var cust_type = ''; var order_value = ''; var order_id = ''; var units_ordered = ''; //--> </script> <script language="javascript" src="https://www.shopzilla.com/css/roi_tracker.js"></script>

I need to populate that script with certain variables on my stores reciept page. The ones i'm looking to populate are the order_value and cust_type variables.

Somehow, I need to have those values automatically populated. The order_value would be the value of the order just made, and the cust_type would be if the customer is a returning customer or a new customer. It says here that the best way to go about that would be to check if their email existed already.

Does anyone know how I would go about doing this? Please help!