[otrs] Freetext field for ITSM's Service

Hello, We are looking at possibilities to setup Services with OTRS and also associate one service with a fixed fee. This is a requirements from our customer and we need to find a way to do this. What I believe is the right way of doing it is to add a freetext field named something like "Fee" which you have to fill in when you create a new "Service". I have tried to figure out how to do this but can't find any good instructions. Could anyone provide me with the steps for how to do this and perhaps point to the sources where I can read about it. Thanks in advance! Best Regards, Niklas