Currently, the Custom fields feature is available in QuickBooks Online Essentials and Plus. With your current plan (Simple Start), you can add the optional credit card surcharge in the Message on invoice field to track your customer transactions.
Go to the +New button.
Enter all the needed information.
Input the "Optional Credit Card Surcharge" in the Message on invoice field.
Click Save and close.
I've attached a screenshot below that shows the fourth to fifth steps.
Moreover, in case you still want to use the Custom fields feature in adding the optional credit card surcharge in your transactions, you'll have to upgrade your plan from Simple Start to either Essentials or Plus. For the detailed steps, kindly refer to this article: Upgrade your QuickBooks Online subscription.