Dotmailer ROI Tracking

[leej]

Hi there,
We have a client who has asked us to implement some ROI tracking for Dotmailer. Personally I haven't had much experience with this however it looks fairly straight forward. The part that I am unsure of is within the tracking it looks like there are ROI markers e.g. A record per each product

Example..

<script type="text/javascript">
_dmTrack("product", "Product name 1");
_dmTrack("product", "Product name 2");
_dmTrack("product", "Product name 3");
_dmTrack("CheckOutAmount", "250");
</script>

Could anybody suggest how I could activate a ROI marker if the customer purchases multiple items. Any help or advice regarding this would be greatly appreciated.