financial transactions with and without commissions
For any application where you are tracking money changing hands, you should always use a Double Entry Accounting system.  Double entry accounting has been the standard for tracking money for over 500 years.  There is no good reason to use any other method.

This entails a TRANSACTION table and a TRANSACTION_DETAIL table.  The transaction table has one record per transaction that includes header information, like the date, the total amount, any categorization attributes or relationships, etc.  

The transaction detail has two or more records per transaction.  These records break down where the money comes from and goes to.  In your case, you might have three records per transaction.  One shows the money coming in from the payment service.  The other two show the money going out to a commission account and to an income account.