For certain marketplaces, ShipStation can resend the confirmation e-mail when you ship an order in the app. If you use the e-mail templates (or your own customized one if you're on a Silver plan or higher), this option will resend the e-mail from ShipStation.
To resend the confirmation e-mail for a shipment (or multiple shipments), first go to the Shipments page.
Now, find the orders for which you want to resend the confirmation e-mail, and select them. Once you've selected the orders, click the Resend Confirmation(s) button.
A small window will display confirming your choice, and then the e-mails will be sent.