how to differentiate types of transactions at bill-to

Hi, I have the following iex question: we are sending various types of invoices, in one operating unit. One type needs to have its own dunning letters output. These particular dunning letters needs to be sent seperate from the other types as well, as a result of legal requirements. So to my opinion there is a conflict in defining the level at which letters need to be sent: bill-to level for the regular invoices and delinquency for the legal ones. But as far as I know this is not possible within one operating unit. Please advise how to solve.