Can I track custom parameter such as affid?

You can track any custom parameters you like such as affid,user_id, id with UTMSimple.
Setting this up is very easy...
Your JavaScript tracker code will look like this..
<!-- Global site UTM Simple Tracking Start-->
<script>
handl_custom_params=[];var handl_js=document.createElement("script");handl_js.setAttribute("src","https://track.utmsimple.com/utm.js?license=<yourkey>"),document.head.appendChild(handl_js),handl_js.onload=function(){/*This is the UTM Simple ready event, you can put any code you'd like to trigger after the script fully loaded here*/};
</script>
<!-- Global site UTM Simple Tracking End-->

As you can notice, there is a parameter called handl_custom_params=[];
This is an array that means it can take multiple values.
Simply add all the custom parameters you'd like to track in there...
handl_custom_params=['affid','user_id','id'];

That's it!