I've had a zip submit that converted @ 88% on a good day. Average over a 2 month period was around 75%. It all comes down to creativity. If you do it well, you really can make a killing on them. But there are times when both the affiliate network and the advertiser are skimming (meaning the pixel doesn't fire) and that's when you'll see much lower conversions. Now dont go accusing your aff network of skimming until you're pre-popping the zip and track all the traffic you send via the prepop.