How to add (and change the font of) the short product description to order page and customer’s new order e-mail [closed]

I don’t know what the font is for the rest of the email but if you can discern that by examining one of the emails sent you’ll want to copy the style rule to this element:

return $item_name . '<br><p class="item-description">' . $excerpt ; '</p>';

As this is an email you’ll have to rely on fonts available to the user, so check the email and locate what fonts are being assigned and then add them like this:

return $item_name . '<br><p class="item-description" style="font-family:WHATEVERTHEFONTIS,Arial,Helvetica,sans-serif;font-size:XXpx;font-weight:XXpx;">' . $excerpt ; '</p>';

In earnest though, you’d be better off overriding the email templates using standard WooCommerce (I’m assuming this is for WooCommerce.) template overrides in your theme/child theme.

https://docs.woocommerce.com/document/template-structure/

That will give you full control over the templates and rather than inline styling will allow you to update the actual <style> at the start of the email.