Postback URL allows you to send the information about conversions to the PPCmate reports. This way you to track ROI on the reports page and optimize campaigns to achieve better results.
You can access Postback URL on the campaign set up page. It will look like this:
Please replace the "return_value_USD" to whatever value you want to use a conversion price. This will later be reflected in the reports.
You can find more information regarding postback and how to use it here: