Apply derived flow configuration dependent on transaction information

I want to apply a variable charge to a TRM transaction. I am aware of the derived flow configuration and have got this to work successfully after updating the SI:Derived flows on the BP. My issue is that the charge is not always applicable; it is dependent on the nature of the transaction. In some cases the charge must be applied but in others not. My understanding is that maintaining the SI on the BP will mean that the charge is always applied.

I would like to be able to influence this by maintaining an indicator/field on the transaction itself but I am not sure if this is possible. I've read about BAdI TPM_TRL_MANIPULATE but I'm not sure if it's applicable in this case.

Does anyone have experience of a similar situation?