If you wish to send a sales receipt to your customer online, you will be required to specify a customer.
In the receipt edit screen, click on Save and send button.

After clicking on the "Save and send" button, a send window will pop up. You can proceed to,
  • Write an e-mail message to your customer, and click "Send". 
  • Download PDF. 

Now you will be re-directed back to the receipt list screen, where you can find the newly sent receipt marked as "Sent".
You can "Edit", "Send", "Download" as PDF, "Duplicate", or "Delete" in its "Action" or use the filters to find other receipts in a certain period and/or with certain number/customer.