add postage to invoice after sold

[ginaperry]

I want to add postage to an item which is sold. Looked in help but can't seem to SELECT ITEM or SPECIFY DETAILS just don't understand how to do this.  To me select means highlight but I can't do that in send invoice section.  Then I have no idea what they mean by Specify Item???

Help, my poor buyer needs invoice to pay so he can get his goods.  Thanks G