Hi Adam!
Thanks for getting in touch! Let me help you with this topic.
As a general rule, you shouldn’t modify any plugins files directly, but instead use one of the methods described in our Themer’s guide to customize our plugins > https://theeventscalendar.com/knowledgebase/themers-guide/
Let me note that we are fairly limited in how much support we can give for custom development questions like that as we state here > https://theeventscalendar.com/knowledgebase/what-support-is-provided-for-license-holders/
That said, we always like helping out and at least point users into the right direction as much possible.
You should try customizing the email.php template file located at /wp-content/plugins/event-tickets/src/views/tickets/ by making a template override or by using the ‘tribe_tickets_ticket_email_ticket_bottom’ action hook provided in it.
I hope it helps. Let me know if you have other questions and I’d be happy to help.
Best,
Victor