You will need to create multiple versions of the same ad zone with different payment models, and chain them together.
For example, if you want to sell a 300x250 ad zone on a CPM and Pay Per Day basis such that pay per day sales
have priority over CPM sales, you can
- Chain the CPM ad zone to the Pay Per day ad zone,
- Install only the Pay Per Day ad zone's tag on your website.
When no Pay Per Day ads are available, the CPM ads will show instead.