how can i add flat amount of tax (Total tax )

How can i add flat amount of tax in invoice. that is not calculate base on items. its fix amount or calculate base on sub_total.  example, 

1 . Items prices are 100  +

2. Shipping Charge  :15 +

3. Tax :7 (its not fix with every items, its should 11, 15 or any)

I'm having the same problem. A shipping company sent me an invoice for tax on an item they shipped. No item, just tax. So, I need an item for tax only that gets added to the tax COA