Invoice - separate optional custom fee donation from event registration fee

Multi-member/group checkout - We are using a Custom Field to collect a donation as a custom fee. The system's Invoice generator calls the entire amount collected the event fee and donations do not count as a qualified medical expense. Is there an easy way for the Custom Fee, if any, to be reported as a separate line item on the invoice?
I realize the normal use case is that custom fee was designed for modifiers to an event registration price and there is no way to know whether a fee should be a price modifier or a separate line item for an independent charge like a donation. The good news is that in our setup, there are no price modifiers. And the custom fee is set to not be discountable in case that flag helps somehow. Ideas?

Thanks for any suggestions.

Hi Duke

Due to the complicated of custom fee fields implement, we have no way to have a separate line in the invoice for each fee field

Tuan

I knew that, I was just exploring if it was possible to break out the summary billing one level using the custom fee.
(Also, The invoice also doesn't show the number of admissions/tickets.)
So I'm exploring working backwards as follows:

$amount = total of registrations and fees before discounts and taxes as currently shown
$fee = [custom_fees]
$members = number of members in group.
$tickets = $amount - $fee

Invoice says:
Qty.................Description ................................................. Amount
$members.....$item_desc ("Tickets to Event")....................$tickets
"-".................."Additional Optional Donation, if any".................$fee
.........................................................Total Charges:..........$amount
.........................................................Discount:
.........................................................Taxes:
.........................................................Total Payment Due:
.........................................................Deposit Received:
.........................................................Remaining Balance

To make this work, just need access to the total custom fee [custom_fees], or the Additional Donation custom field [custom_field_12] if the value was saved to the database. I understand it prevents using custom fee for any other purpose in my instance of events booking - I just want a way to recognize that there was a donation added so we can thank the donor appropriately. Is it practical to expose [custom_fees] as one more mergefield? (I'm at the edge of my Joomla PHP skills.)