Your explanation of whatever you think is wrong is not clear. You third screen shot says you have to check boxes to use the second line. But that is not true. The second line is activated by checking the Add line box. In your second screen shot, you have told the program to ignore quantities and unit prices. But if you then put in a line item with a quantity and unit price, the columns are activated and you have to use them for all line items.
Think of it this way: At your step 2, the program does not know the quantity or unit price of the receipt being applied against your customer’s Accounts receivable balance. It only knows the amount to post to that account. After all, there is no quantity or a unit price for a receivable. The customer owes what the customer owes, so there is only an outstanding balance.
When you add lines that demand that information, you are changing the calculation routines in the program, so you have to supply the information. One way around this is not to use a non-inventory item, but only enter a second line item with the amount. That way, there is no quantity or unit price anywhere. In a way, the slight inconvenience you report is the result of treating a fee as an item. Items need quantities, units, prices, etc. A fee just needs an amount.