If an invoice has been created but a description or item needs to show up in a certain section among all the lines, you do not need to remove line items or recreate your invoice to accommodate this. There is a capability to insert an item among the lines that have already been entered on the invoice.
First, select the Item field below the line where you would like the new line to be. Now press the Insert key on your keyboard and the new line should appear above.
Now a new item can be entered among the others.