Hi Allison,

Thanks for posting in the Community!

If you are using the QBO Plus and Advance plan, you can enter billable expenses in QuickBooks. First, let's turn on billable expense in your settings.

1. Click on Settings and choose Account & Settings.
2. Click on Expenses tab.
3. Click Bills and expenses and click on Edit.
4. Turn on Make expenses and items billable.
5. Click on Save.

Next, you can set a product or service item for the per diem you've received. To create products/service:

1. Click on Settings and choose Products and services.
2. Select product type like Service or non-inventory.
3. Enter name like Per diem.
4. Enter details in the other field.
5. Click on Save.

Once done, you can create and add billable expenses to invoices.

1. Click on + New.
2. Click on Invoice.
3. Click Customer from the dropdown menu.
4. Click Add on the billable expense you wanted to charge.
5. Click on Save and close.

To learn more about the billable expense option in QBO, check this link Enter billable expenses in QBO. 

Feel free to reply to this thread if you have other questions with QBO.