Thank you for your post.
You are very close to solving it yourself! Here is the formula:
Case (Order_Status = "shipped" and Order_Date > Date_Minus30 ; Order_Quantity ; "" )
That is, If the status "shipped" and the Order_Date is greater than (more recent) than "Date_Minus30", then use Order_Quantity. Otherwise, leave it blank. Does that make sense?
It works - perfect - thank you!