In addition to being able to charge your clients hourly, a fixed price, and by retainer, you can add per item pricing to a project so you can have a separate cost for each task within a project.
Click on any project and then edit to bring up its settings. Under "Project fee type" select "Per item."
Now when you add a task to this project, you will see the option to access your product/service library. Add an item from your product or service library or create a new product or service.
Product and service library
Each time you create a new item in your product and service library, it will be automatically saved for future use. This allows you to charge by word for a blog post, by asset you create, or set different fixed rate fees per each task in a project.
You can also set up different hourly rates per task in a project by using the "Billed by the hour" check box and using different rates per the work you are doing.
Edit pricing using the edit pencil. Remove items with the trash can icon.
Other uses for the product/service library
Save this project plus the associated tasks and fees as a template for future use.
You can also use this library to manually add non-tracked items to your invoice. Create a new invoice and choose "Add new line item." This will pull up your product and service library and allow you to choose one of your products or services at your saved rate.
Edit and manage your product and service library from workspace settings > products & services.