VAT invoices are for accounting needs primarily for those who are registered VAT payers.
Go to the Billing section on your Printful dashboard and click “Monthly Invoices.”
You'll be able to see invoices for your UK orders (this will have Printful's UK VAT ID), and an invoice for all other EU orders (this will have Printful's Latvia VAT ID).
These are available on the 10th of every month, for the previous month. They're only available for orders that have been delivered.