JavaScript version of integration code is generally recommended, because it allows using advanced tracking techniques.

Open your order confirmation or "thank you for order" page template, and put the following code somewhere onto the page

<script id="pap_x2s6df8d" src="https://amik.postaffiliatepro.com/scripts/trackjs.js" type="text/javascript"></script>
<script type="text/javascript">
PostAffTracker.setAccountId('default1');
var sale = PostAffTracker.createSale();
sale.setTotalCost('120.50');
sale.setOrderID('ORD_12345XYZ');

PostAffTracker.register();
</script>

where the parameters in setTotalCost() and setOrderID() should be replaced with correct values.
Total Cost (mandatory for % commissions) - price of order with discount but without shipping and tax
Order ID - can be your unique generated order ID to cross-check the sale.