You've probably already found it by now but I put the URL for Shipstation below. It's a royal pain to find it through Paypal and I've never actually been able to find it going through the Paypal website. I'm sure there is probably a way but they certainly don't make it easy. To find it, I had to do a Google search asking where Paypal shipping was located and some article laid it out with how to get there.
https://www.shipstation.com/