Why can I not add freight to a parts invoice?

In the Parts Sale screen, there is an area to add freight to your invoices.

If this is greyed out, the specific charge type you are using has not been enabled to add freight.

1. Click on System in the top left of the Parts Module.
2. Click Manage.
3. Click Parts/Sale Charge Types.
4. Under the Charge Types tab, ensure the Allow Invoices to Freight Amounts check box is ticked.