To test one-time receipt emails, go to your Campaigns page and click on the "Edit Campaign" button of the campaign you wish to test:
Next, click Receipt Emails on the left-hand panel of the following page.
Then, scroll down, and click on Send test Emails
After adding the emails, click on "Send Email"
Once you click the "Send Email" button, the test emails will be sent.
You can now also test sending receipt emails for your Events. Learn more about it here:
How do I edit the events receipt?