'Within this week'?
I would grade that as an 'F'. I would request that, until they are able to type a coherent letter, you are totally fine with your child having a sandwich and a piece of fruit.
It seems 'isolation' is a term or process used to push you into paying and so is presumably a punishment. The first thing I would be doing is asking them, why, as 'teacher' do they deem it necessary to vicariously punish a child for circumstances completely out of the child's control. It seems the threat of isolation (or detention) is being used by nothing other than a form of blackmail which, in English and Welsh law, is illegal.
I would also be asking the school what exactly they expect the child to learn from being placed into isolation. They are blatantly affecting an innocent child's social being for money.
I would also in very polite terms, explain that if they dare threaten your child's happiness and/or innocence for monetary purposes again, they will be receiving a Solicitors letter.