Hi Paul,
I just have an idea about the instance code. I supposed that the code will be unique through my sales process. Maybe, order id is a proper ones. I don't know if Squrespace have some data field to take down these kind of customized code in its database. Or, I will subscribe to the order event via Webhook API, and generate that code in my system and connect the order ID with the code. This is just an idea, and has not verified yet.