Tickera itself does not support invoicing in traditional sense. However, as it is sending emails once when the customer places their order and again when their order is completed and contents of these emails can be modified any way you like within Tickera Settings -> Email tab, you can use content of one of these emails as receipt. You can edit the content of these emails using standard WordPress/HTML editing as well as using Tickera specific placeholders.
Also, given that Tickera has a seamless integration with WooCommerce via our Bridge for WooCommerce add-on which, essentially, allows you to create and sell Tickera tickets like any other WooCommerce product, you can use third party plugins for WooCommerce that can provide you with the functionality of sending standard invoices once the customer finishes their purchase. There are several of such plugins on the market but we suggest you using the one found here for which we have also created an add-on that allows you to place attendee/ticket information right on the PDF invoice. You can download this add-on here.