PopCash is already experienced popunder network being paying publishers over 2 years. What I could see is that this company is located in Romania and has good promotions including facebook or linkedin. Almost all publishers are accepted in the network.
They have very clean ads and they test advertiser landing pages multiple times per day. There is intuitive and very easy to use user admin interface. Platfor offers many targeting & optimizing possibilities and various traffic sources.
Their minimum payment is $10 and this is sent via Paypal, Payza or Paxum.