At checkout, after entering your shipping information and choosing your preferred payment method, you will find a box to tick with the words "Do you need an invoice?"

By ticking it, fields will appear that you can fill in with the company header, address, VAT number and any SDI code. Alternatively, you can write to sending all your data. Remember that, according to Italian tax legislation, it is not permitted to issue an invoice later than the twelfth day from the date of purchase.