You may even want to tell them that their feature will be on a separate branch and any maintenance, including merges from the mainline code, will also be for a

You may even want to tell them that their feature will be on a separate branch and any maintenance, including merges from the mainline code, will also be for a fee. Also, if they want to see your reasoning why you don't like the proposed feature, they will also have to pay up, because it costs you time to write that down.