To test one-time receipt emails, go to your Campaigns page and click on the edit button of the campaign you wish to test:
On the next page, click on the edit button of the section "Email Receipt":
Then, scroll down, and enable the option "Display email testing options":
Once you enable this, you will be able to add up to three different emails. After adding the emails, click on "Send Test Emails":
Once you click the "Send Test Email" button, the test emails will be sent.