Well done, you sold 50,000 Baby Wee Wee dolls (hey, we're just picking a product at random here). Now what? You've acquired 50,000 Baby Wee Wee fans, not 50,000 customers. They only bought from you because they're big fans of (all too) anatomically correct dolls and you happened to be around to feed their need.
Wouldn't it have been better if you'd sold a handful of Baby Wee Wees, plus maybe some Breast Milk Baby dolls, some Pole Dancing dolls and... um, maybe some products that weren't massively inappropriate? Remember: volume sales needn't only mean selling large quantities of one single product SKU - in fact, you're never going to grow your brand or acquire loyal new customers if that's all your marketing sets about to do.
That's one of the reasons daily deal promotions don't work: you blow your entire quarterly budget on promoting one product and, at best, acquiring a very limited type of customer. Far better to spend a much smaller amount of money over a longer time, and acquire a wide range of customers who actually care about your brand and your products.
That's why we tend not to charge per Co-buy. Instead,