I would argue that storing snapshots of a bill, separately from the actual bill data, is a poor design decision.  If a bill is to remain the same, then the underlying data should remain the same.  If adjustments need to be made to a bill, they should be made via debit or credit memo; or via the issuance of a new and updated bill that supercedes the prior bill.

So far it sounds to me as if the new UDTs are simply a way for non-SQL developers to force a non-SQL solution onto SQL.