Within Teamwork, you can add expenses to invoices on your projects.
Go to the Billing area of your project and select the Unbilled Expenses subsection.

You can use the filters at the top of the Unbilled Expenses subsection to refine the expenses shown in the results.

Check the boxes to the left of each expense you want to add to the invoice and click Move Items.

You can then choose the invoice you want to move the expenses to from the dropdown menu. Click Move Expenses to save your changes.

You can also move one or multiple expenses to an invoice using drag and drop.

Note: Once an invoice has been completed, any attached expenses will be marked as billed.
For more information, see: Adding an Invoice