We have many many clients who want shipping notifications sent to multiple people in their organizations. Or, with some clients, we want a notification sent to both us and them.

ShipStation currently allows on email address, despite the fact that the email address field can contain at least 757 characters (I didn't feel the need to check for more than that).

All of the email systems I'm familiar with allow for multiple email addresses to be listed by using a separator, usually a comma or a semi-colon, to distinguish between unique email addresses, like this:
"name1@company.com;name2@company.com;name3@company.com" (this includes emails sent from QuickBooks, which is definitely not primarily an email service).

Now, I realize that there is an Automation Rule option for sending emails that accepts multiple email addresses. However, the rule is processed upon importation of the the order, so it will not work for a Shipping notification.