Hi Nicholas,
Unfortunately there isn't a feature that currently supports auto-emailing customer receipts. There is a feature for auto-emailing customer's their invoices or statements however, please click on the link below to view a guide on this.
https://www.clearbooks.co.uk/support/guides/email-sales/automatically-send-emails-when-an-invoice-is-created
Therefore, this would need to be converted into an idea and as with all our ideas the more popular it is the more likely the developers are to consider it for implementation as a main feature.
All the best,
Shaz