We have the same issue with Amazon orders. We don't sell on eBay. Etsy
and Shopify work but all shipping fees are $0 for Amazon orders.
Shipstation has been horrible this year. The new UI is terrible and
there are way too many glitches.
So, I just tried again and this time it worked. It feels like it was a
problem on Etsy's side. Maybe? Anyway, resetting the marketplace
connection no longer gave me a 404 error.