You have the option of sending an invoice to an external email address.

STEP 1:

Select any invoice you want to send to an external email address.

STEP 2:
On the right-hand side, you'll see the option to '+ Add external user email address.' Once you click on that, you'll be able to enter an external user email by clicking on the '+ Add External User Email.'

STEP 3:
You will note that an email body will prefill, however are able to add free text & edit this email to your preference.

STEP 4:
Click 'Send email,' and the invoice will be sent to the email address you entered.