Processing Payments with transaction fees (like credit card processing, or PayPal)

[Buddha Buck]

I receive payments for invoices via PayPal.  PayPal takes a
transaction fee when this happens.

My current procedure (if Bob buys $20 of services) is as follows:

1. Create an invoice to Bob for $20 of services.
2. Wait for Bob to pay $20 via PayPal
3. Use "Process payment" to process the payment.  This creates a
transaction in my PayPal account for $20.
4a. Add a "Transaction Fees" transaction to the PayPal account of
"credit PayPal $0.88 debit Expense:Bank Fees $0.88"; OR
4b. Manually edit the splits for this transaction so that instead of
"debit PayPal $20 credit A/R $20" it becomes "debit PayPal $19.12
debit Expense:Bank Fees $0.88 credit A/R $20.

I like the 4b approach because it more closely resembles what I see on
my statements from PayPal, and because it clearly ties the transaction
fee to the payment in the register.

But I don't like doing it.  Is there a better way?  Is there a way to
get the "Process Payment" dialog to ask about transaction fees when
processing the payment?  Better still, is there a way to enter the
transaction fee formula somewhere so that all payments processed using
the PayPal account automatically get the proper split?