Groupon Checkouts

I'm trying to set up a cart and everything seems fine however I have one major question.

How do I go about processing Groupon buys? If I have product X on Groupon, people buy it meaning they fill out the account/billing/shipping and pay on Groupon's site. They then need to fill out their information Open Cart and checkout without paying again. How do I verify they paid via Groupon during the check-out process?

Maybe I'm going about this wrong or just don't fully understand how Groupon works. My boss has done Groupon deals but only sold a service not a product. We're moving into product sales as well now.

I take it no one has ever sold items via Groupon and then been redirected over to OpenCart to finish up the checkout process?