Opencart 3.0.2 and Shipstation intermittent problem

I'm using Shipstation integrated with Opencart 3.0.2 for most of the time it does actually work. However, it depends on the payment method and source. Payments made through the regular credit/debit card gateway go through, but PayPal paid orders do not. At the same time orders brought in from eBay via Openbay also do not pass through. Has anyone else had the same issue and more importantly, found a solution?
Fo the record, the extension in provided by Shipstation but they're not interested in supporting it.

This topic has now been moved to the OpenCart 3.0 Support > General Support section of the forum.