I think it's bad customer service to force someone to Google for a coupon code; you're also running a risk (as a site owner) that the person will simply abandon their cart.

See also this thread here: https://news.ycombinator.com/item?id=6204261

I like petercooper's idea of pre-populating the coupon field with a default code.

I wonder if a competitor could grab the sale by purchasing ads against "<Your Company> Coupons" claiming to be 20% cheaper or something.

They could. Accept they won't be buying them from Google since they have ridiculous trademark rules. You can't bid on things like that now.

Ah fair enough, pretty pointless now then. Google have done a great job forcing the legitimisation of a very gray market.

I can't remember the last time I found a decent coupon code there.

Really? You must not do much online shopping. There is tons of times there isn't any coupon code that applies to what I'm buying and then I just feel kinda cheated. Just a little. Then there's tons of times there is no coupons at all for the site, maybe just like $10 off your $100 purchase of <very specific product> and that's it.

Well, to give specific examples of coupons I found there, I usually get 20% off when I order Papa Johns, and I got 2 months free web hosting from Digital Ocean. For Amazon, you're right; they don't usually have anything good for Amazon. For smaller places, my experience has been that they have nice coupon codes if they exist.