When you sign up for bellbot.com, it gives you a small Javascript snippet. You embed that snippet in your "thank you!" page (or whatever page your users see after they do stuff on your site).

Every time a user hits that page, you'll hear a beep on your end if you're logged into bellbot.

It sounds like your place the js file on some post-purchase page. That way when they request it you can be pretty sure they just completed the transaction.