Fantastic Kevin, thank you my friend. Big learning moment for me and I won't be forgetting the AND statement after this. (hopefully as I am old ha ha).

The Switch() function still won't work if all conditions are false, which would be the case if the due date is not yet 30 days old, the invoice still unpaid, and the due date is before today's date.  For instance, if the today's date is 4/24/23, and the due date is only 23 days ago (4/1/23), and [IsPaid] is also false, then all 3 conditions in the Switch() function will be false.  In that case, Switch() will return null instead.  You need to add one last condition that is guaranteed to be true, and add a return value for that.  This is necessary for Switch() in general:

    =Switch([OrderT.DueDate]<Date()-30 And Not [IsPaid], "Over 30 Days Past Due", [IsPaid], "Has Been Paid", [OrderT.DueDate]>=Date(), "Payment Not Due", True, "Less Than 30 Days Past Due")

If you find this cumbersome, you may rearrange the order of the conditions.  I would put the [IsPaid] condition in front, then check if it is over or less than 30 days past due:

   =Switch([IsPaid], "Paid", [OrderT.DueDate]>=Date(), "Payment Not Due", [OrderT.DueDate]<Date()-30, "Over 30 Days Past Due", True, "Less Than 30 Days Past Due")

The Switch() function looks at conditions from left to right, and as soon as a True condition is found, it stops looking at subsequent conditions.  So the most "decisive" condition should always be in front.