According to the Italian tax regulator, a revenue stamp (in IT Marca da bollo) should be added to all invoices, in which you have indicated VAT as 0% and where the total amount of the invoice is greater than €77.47.
Below are some steps on how you can add this to your invoice.
Firstly, a revenue stamp must be enabled within the invoicing settings. In the top right corner of your main dashboard click on the name of your company and choose "Settings" from the drop-down menu.
On the left side of the screen, you can see the settings for your personal profile and the company profile. From here, click on the option for "Invoicing".
Once selected, scroll down the page until you see the option "Enable revenue stamp". Click here to activate the function, and Revenue stamp will then become enabled once the button switches to blue.
Finally, to add a stamp to the invoice, open the invoice you need from the main dashboard. Then, in the bottom right corner, click on "Apply revenue stamp".
You can choose to pay for it yourself, or to put it at the expense of your customer. If you are unsure how to pay for the revenue stamp, please consult with your accountant.
Now your invoice is complete, and ready to be sent to your client.