Unfortunately, it is not possible to add an extra charge to only one line on the Sales Order.
A potential work around for this is to include the charge in the 'Gross Price Each' on the Call Off. You can do this by doing the following:
Click Sales then click Sales Order Menu.
Click Sales Order Maintenance then click F5 on your Sales Order.
Click F5 on Order Quantity then click F5 on Gross Price Each.
Include the price of the charge spread across the Gross Price Each per Quantity Outstanding.
For example, if you require an additional charge for £10 and you have 10 parts on the Call Off, you will need to add an additional £1 onto the Gross Price Each.