Not as a standard. Will suggest you a before load user events in where you can drive billable items sublist. But I'm not sure if the quantity is enabled and you can change it so you need to try

Hi, thanks for the answer. We have actually tried it, the script is giving this error: You cannot edit charge lines on invoices, cash sales, credit memos, or cash refunds. So I think there is nothing we can do.

Also in before load? Good to know. Is the sublist editable in the UI?

Before load we cannot access the rows and the code, it somehow loads the rows based on milestone rule after we create the invoice record. Only then it starts loading the rows. It could be possible if we can access the code, or we could also receive the same error again. Those rows that are created based on milestone rule are not editable in the UI. We can only create completely new rows.

Yes, it should be only a APPLY (checked/unchecked) available for update