...
Please note that you need to setup straight away a payment fee within a overhead table fee, you cannot add it after having created it (MMI to recheck in details this)
The setup needs to be done at the insitution level where you create first the overheads you want to have and the overheads table combining the overheads you just created. (each overheads can have a different VAT) Then you simply access your sales channel and assign the overheads table.