Setcom integrates using Auto-post callback.
Also,make sure you don't already use Auto-post for another purpose, such assome kind of digital delivery or membership registration.

To integrate with Setcom, you need Setcom plugin which is distributed with PAP by default.

1.

Configure plugin

First you must activate Setcom plugin in PAP->Start menu->Plugins. After plugin is activated you must configure it. Click Configure button. You must set your Merchant username, password which you are using to login to your merchant panel. Merchant identifier is located in your Setcom merchant panel top menu->My account->Overview tab. Leave custom value separator blank.

2.

SetUp your Setcom settings

Login to your Setcom merchant panel. In top menu->Profile->Selling Preferences select Website Payment Preferences. Make sure that Auto-post is set to on and that Auto-post URL is set to http://URL_TO_PostAffiliatePro/plugins/Setcom/setcom.php.
Do not forget save your settings.

3.

Update Buttons

Now add the following code into EVERY Setcom (Buy now, Add to cart and/or Checkout) button form

<input type="hidden" name="MerchCustom" value="" id="pap_dx8vc2s5">

For example:

<form action="https://www.setcom.com/secure/index.cfm" method="post">
  <input type="hidden" name="buttonaction" value="buynow">
  <input type="hidden" name="merchantidentifier" value="12345678920">
  <input type="hidden" name="liddesc" value="auticko">
  <input type="hidden" name="lidsku" value="a123">
  <input type="hidden" name="lidprice" value="150.00">
  <input type="hidden" name="lidqty" value="1">
  <input type="hidden" name="CurrencyAlphaCode" value="ZAR">
  <input type="hidden" name="LIDExpiry" value="1">
  <input type="hidden" name="ShippingRequired" value="0">
  <input type="hidden" name="IsVoucher" value="0">
  <input type="hidden" name="Option1Name" value="custom">
  <input type="hidden" name="Option1Value" value="oldCustom">
  <input type="hidden" name="MerchCustom" value="" id="pap_dx8vc2s5">
  <input type="image" src="https://www.setcom.com/www/graphics/cartbuttons/bn6.gif">
</form>

After this form you must insert:
<script id="pap_x2s6df8d" src="http://URL_TO_PostAffiliatePro/scripts/notifysale.php" type="text/javascript">
</script>


You can find more detailed info about Setcom integration in Setcom Implementation Guide