You may not get many people disclosing specific arrangements as it could be considered quite sensitive data.
As a general rule i would say that if an undertaker actually takes the order from the client and passes it on to you, then it would be appropriate to allow a commission, just as you would a relay service. If it is a case of recommending you to a client in preference to say another florist, then it really is up to you how much you value the referrals. It is probably worth having a chat with the undertaker and coming to some sort of arrangement for both cases.