Official comment
I believe that can all be done using Google Analytics.
See this article about Google Adwords conversion tracking: https://support.google.com/google-ads/answer/6095821
The "conversion tracking tag" they talk about at the bottom half of that article would go on the thank you page for the purchase.If it's an ONTRAPORT landing page you could place the code in either the header or footer code sections (on the settings page for the landing page).