How to mark items as "invoiced"?

I create an invoice including some billable items/lines.
I later discover I’ve missed one billable item, so I edit the invoice to add that item.
when the Invoice is completed most of the included items are listed as “invoiced”, but the item that was added later remains “uninvoiced”.
I can’t find any way to mark that item as “invoiced”, how can I correct this?

Please note:
I am not an accountant or book-keeper, so please try to reply in plain English!
:thinking: : ~}

Read the Guide: Invoice billable expenses | Manager. See the final section, Adding billable expenses to a sales invoice.

Thank you Tut.
I think I know someone who can walk through it with me and translate.