Last month I used InfluAds (think along the lines of "The Deck") which paid on a CPM model.

This month I've switched to a flat rate for a 30 day run. I'm selling ads directly (AppSumo and MediaTemple), and through BuySellAds (no one has bought through this, but I think it's because I'm charging more).

If I know the sponsor well, I have no problem charging at the end of the month. If you have any more questions feel free to shout!