It doesn't matter how you decide to buy/pay for your impressions because it gets normalized by their ad system to an effective eCPM (revenue per 1000 impression

It doesn't matter how you decide to buy/pay for your impressions because it gets normalized by their ad system to an effective eCPM (revenue per 1000 impressions). Let's say you only want to pay via a "click-through-and-buy", after a certain number of impressions and clicks they will have a benchmark for how much in commissions were paid. They take that number, divide by impressions/clicks, and then have an eCPM to rank you by. If the eCPM achieved by allocating impressions to your campaign is less than via traditional CPM or CPC based buys then you will get lower allocation and thus the system optimizes you out of inventory.