If you wish to have a non-taxable item in an order display without tax,  then the following steps will show you how.

Have "Enable Tax on Order" checked in the Tax Setup (under Orders Menu), and "Selected Price List Items" checked for the relevant Tax Option, then

1. Go to Price List Setup (under the Orders Menu),
2. Select Order Adjustments tab (for example)
   
3. Select the item (eg Sitting Fee)
4. Click the Change button (top right)
5. Uncheck the "Taxable" checkbox.
6. Click the Change Item button.
7. Click the Close button.

Tax on Custom Extras

If you have enabled tax on orders and "Add tax to Selected Items" is selected (as mentioned above), then a checkbox will be shown next to the Custom Extras line so you can select whether tax is applicable to this item (Place Order --> Add with Extras).