Modules vs Work Types

Afternoon

We are relatively new to Intervals but are using it with our agency in London.

We currently have the default modules and worktypes configured in our account. Our Project Managers own the projects with the developers and designers being resources. However, some tasks within a project are billable and some are not.

An example would be a project for a specific client that is "support" tasks or bugs. We have an agreed number of hours pre-paid by the client per month and anything over that is chargeable. In addition, some work may be billable and some not.

To get around this, we've added modules for non-billable design and development against tasks. This however contradicts what modules are used for and I think we are meant to use time information for this. However, the developers have no idea what is billable or not and therefore cannot make that decision.

Another way to solve this would be worktype but that again is linked to time not the task itself and therefore forces the decision back to the developers.

So, my question is, does anyone have a good way of solving this other than using modules ??