Hey @PeterCrosbys
Thanks for reaching out! Sadly, we do not have this feature currently - I've submitted it as a proposal to our product team to consider implementing in the future.
In the meantime, I was able to find two workarounds that may work instead. First, you could take a screenshot of the email and save it as a PDF. You can then include a link to the PDF which will be hosted on your website. From there, they could print the email.
Additionally, you could include a web link. When a customer clicks on the web view link in an email, it will open to a new tab/window and from there, they could print the page.
More information on adding a web link here: Template Tags and Variable Syntax
Hope this helps!