Billetto has a function called "Generate tickets." This allows you to send free event tickets to sponsors, members, and so. These will look like regular tickets, but the price will be stated as "0 GBP" when customers receive them.
You find this function under by going to My events -> Manage -> Tickets -> Generate Tickets (at the top of the ticket list).
You then pick the ticket(s) you want to send to your customer and type in their name, email, and (optionally) telephone number. The customer will then receive the ticket(s) in their email inbox.
Note: You can send 20 tickets at a time using this function. Remember that you're generating a free ticket, so even though you pick one of your paid tickets to generate, the customer will not be charged.