Ask HN: How does Groupon make their coupons to such a wide variety of retailers?

So, as you all know Groupon offers coupons to all kinds of online and offline retailers. Each have different software, and hardware (in some cases) for accepting payments and orders. how Groupon makes integration accessible on business' side, on a technical level. How does a business incorporate the Groupon sale into their bookeeping and inventory management. And what does Groupon do to facilitate this?