When guests buy tickets via your online Giving Center, they automatically receive an e-mailed receipt detailing their ticket purchase. You can include special instructions about attending the event, receiving physical tickets, etc.
This ticketing-specific emailed receipt can also be manually sent when you sell tickets to supporters via the back end in Planner > Tickets > Sell Tickets - make sure to click the "Email Receipt button on the final screen of your sale:
The ticketing email receipt's customizable message can be updated under Tickets > Setup Tickets > Receipt Message tab. Enter your text and click Save.
The custom message will appear on the emailed receipt, followed by a detailed list of purchased tickets and ticket options, and any tax information filled out under Setup > Preferences > Organization.