Change the fees section to a % option

In store settings > operations there is a ‘fees’ section which shows a ‘delivery fee’ and a ‘fixed charge’ we use this fixed charge as our ‘service charge’ and charge 50p on every order to help with the SK fees, but would like this to be a % of the order total

As if your spending £5 a 50p fixed fee is a lot where as if you send £50 50p isn’t much,

Spoke to SK help and they suggest changing the Tips section to % but I want customer to pay the fixed charge % as described above, and able to leave a tip of there own if they wish so,