Balance Due (before other fees) Fee changes when another Line Item is Added

Hello Tessitura! 

I have an interesting conundrum and I am struggling to find a solution. One of the consortia members processes subscription exchanges in a Sub Exchange MOS. Their current business practice is to process any additional regular priced tickets in a Phone Sale MOS. However, there have been some issues with folks remembering to actually changing their MOS. So, the proposed solution is to add the "tSubAdd" price type to the Sub Exchange MOS so these additional tickets can be added in the same order. 

The conundrum is when a higher priced ticket is exchanged to a lower price ticket. There is the Balance Due (before other fees) Fee to capture the negative balance. When the additional line item is added, this fee is automatically applied to the balance due for the additional ticket. Is there a setting in the fee to prevent this from happening that I'm just missing? Some way to keep the Balance Due Fee "static" so that the difference is claimed? The phone fee on the "tSubAdd" fires fine. 

Ideas I've Had/Tried:

• Change the fee type to Balance Due (after other fees). Doesn't work
• Set the User Can Edit Amount to Never - still happens.
• Have the user override the Balance Due fee - this technically works, but if having them remember to change their MOS is a big ask, then having them override a fee is not ideal
• Set a message only pricing rule - sure but the one that already fires about parking gets ignored too 

This may be the reason the original business rule about changing MOS was in place, but I figured I'd ask here first! Thanks for any and all help/ideas/solutions!