You can add a custom message to the receipt that gets automatically sent to your supporters. To do this, you'll go the Campaign Editor > Campaign Settings > Receipt Settings. Here you'll see sections to add a custom message to all of the actions that supporters can take for a particular campaign.
You'll want to add a custom note based on the activity, since those who submit a One Time donation can get a different message than those who submit a Monthly donation, or those who volunteer, etc. If someone signs up for the mailing list while completing a transaction on Flipcause, they will get a separate confirmation. The Mailing List Sign-Up option is where you can customize a message to thank them for signing up.
The custom message will appear in the area of red text that you can see in this example, with all automatically-generated details about the transaction below. All receipts, by default, contain all the legally-required information such as tax deductible amount, contact info for your organization, and the disclaimer in case of donations that no goods or services were exchanged.
Once you've entered the custom messages, don't forget to click Save!