There are 3 types of tasks in Paymo:
Time based
Flat rate
Non-billable
By default, tasks inherit their billing type based on the project. This means that:
For a Time and materials project, all tasks will be time-based.
For a Flat rate project, the task type isn’t relevant, because a flat-rate project cost is independent of tasks, so tasks are marked as non-billable.
For a Non-billable project, all tasks will be non-billable as well.
NOTE: You can change the billing type of tasks only for Time and materials projects. This means you can have a time and materials project and have time-based, flat rate, and non-billable tasks in the same project.
In order to have flat rate tasks, you need to: set the project to Time and materials → Set the billing priority to tasks → Change the task billing type to flat rate.
To change the task type you can either hover over the task in List → Full task details and change the billing type or do it from the billing type columns inside the task Table View.