As far as I know, they pay for all impressions, at least that's what I gathered from when I was using them. Although I wasn't a fan of the popups, because some of them contained viruses.
Yup, it counts uniques, and it doesn't even do a very good job of that. Check out my other post on the topic here http://forums.digitalpoint.com/showthread.php?t=135467&highlight=paypopup Tom
From Paypopup's FAQ: :: Payment :: Q. How much do I get paid? A. Standard PayPopup.com payment rate is up to USD$1.80 per 1,000 RAW impressions. Actual campaign rates vary with website performance and market conditions. Paypopup.com Current payment Structure: Now their CPM for China is $0.25 CPM compared to US $1.80 and UK/OZ/CA are like $1.50 or something on average. They do say RAW impressions not UNIQUES. I wonder how many popups they give to each visitor and how often?
The Paypopup interface allows you to decide how often the ads show up. You can set no cap, or a cap of up to 8 days. Most people probably go on the lower end of this scale, and some probably leave the cap off, so you might get multiple ads in surfing through those sites. The good news is that in our experiences with Paypopup, we never got a multi-pop ad. So they might show ads often, but at least they're not too persistent. Tom