If you would like to add a fixed tax amount to your invoice, all you have to do is click the "Tax" field on the document line where you wish to apply a fixed tax:

FixedTax1

This will prompt a sidebar. At the bottom of the list, after all the percentage taxes, you will find the option to enter a fixed tax amount. Clicking on this will take you the manual tax addition sidebar:

FixedTax2 3

The following fields need to be filled in order for you to finish adding your tax manually:
  1. Tax type
  2. Tax rate
  3. Tax amount
Note that you can also add a Tax label, but that is not mandatory. Once all the necessary information is there, simply click "Apply tax", and that tax is added to your invoice line!



You can also add a fixed tax amount on your whole invoice, without having to split it on each line. To do so, click the "Add header charge, discount or tax" link next to the "Add new line" button, and select "Fixed tax" from the drop-down:



Fixed_tax_Header



Doing this means you will effectively apply a fixed tax amount at header level, i.e. on the entire invoice, which will then remove all line taxes you might have entered on the invoice before:



Fixed_tax_header_message



If you choose to add a fixed tax amount for the whole invoice, you will then see a fixed tax line. Click the tax field and enter your fixed tax as you would for a line level fixed tax.