Deal aggregator iframe tracking

I noticed a deal aggregator site (specifically onespout.com) uses an iframe to show the site where you are actually getting the deal from. This means you are on the aggregator's site t开发者_StackOverflow社区he entire time. I was wondering how you the aggregator site can interact with that iframe so you know when a purchase is complete and such. I know this is a little vague but thanks in advance.