Login to your control panel and navigate to Design -> Theme Files/Edit HTML & CSS and open the invoice_print.html file.
Paste the following code at the bottom of the file:
Save your changes, and test the modification by printing an invoice.
How It Works
The code modifies the invoice by adding a new tax line with a value of zero after the grand total. It will only add this line if the invoice does not have a tax. In the case your invoice does have a tax amount then it will not add the line.
If the tax line does not show up on the invoice... then double check the code. Make sure you copied it exactly as shown in the instructions. Be especially sure that the code contains the first line: $(document).ready(function(). If the code was copied correctly then it most likely is an issue with modifications done to the printable invoice template. In that case you will need to contact us to come up with a solution.