Hi sambates123,
did you create an account under “Merchant and Online Payment Service Accounts” for iZettle? If not you should do it. Inside this account you you can then create an Income entry (the payment from your client), an Outgoing payment to your bank account (which you link/tag to the payment from iZettle to your normal bank account) and another outgoing payment to the supplier “iZettle” which you have to create and tag this payment to a invoice to this supplier (iZettle)
You can find more info here:
I hope this helps