I have been trying for over a month to get it to work. Can't get a response from shipstation support, have paid 2 freelancers and iextend (they are still looking at it, but their first effort didn't make it work). It is installed, connects from shipstation, but will not change status to enabled or show on front-end. If anyone has any insights, I am getting desperate.
Alternately, is there another service like this that works with OC3, and ideally connects to stamps.com?
I'm not entirely sure if it's comparable, but I have a Shipwire Integration extension that works on OpenCart 3, so if Shipwire will work for your needs, then that may be a good alternative. Feel free to take a look at the screenshots and demo site, and if you're interested let me know at www.getclearthinking.com/contact if you have any further questions.