Hi,
I'm taking over Philippe here. He is away, on holidays for a while.
The plugin will work fine on PHP 8.1. No need to upgrade or downgrade your PHP.
That error is with the code:
preg_match('#SS-UserName=([^&]+)#i', $path, $matches);
$shipUser = $matches[1];
in the ShipStation plugin. This code retrieves the username from the URL when ShipStation contacts the website to retrieve the orders data.
The fact that you get that warning message indicates that the code cannot find the SS-UserName parameter in the URL used by ShipStation to contact the plugin. So it points at potentially a missconfiguration of the Marketplace Connection in your ShipStation account.
I went there with the access you provided before to Philippe. I can't see the details already configured for the Revel Prints store, as ShipStation doesn't offer that capability. I can only reconfigure it.
So I thought I might do that.
However, when I try to login on the backend of the revel prints website with the access you provided before to Philippe, I'm not able to login. I'm getting the error message "Username and password do not match or you do not have an account yet.".
So I can't verify that what I would configure on ShipStation is the same as what you have in the settings of the ShipStation account.
Looking into it, I can see that you provided that backend access 3 months ago. So I suppose you must have disabled it since then.
Could you please reinstate it, or provide a new backend access via our contact form ? That way, I'll be able to look further into it :
www.hikashop.com/support/contact-us.html