Edit Invoice Line Items
Editing line items lets you correct what a job is going to charge for before it is invoiced. On a work order's line items you can add an item from the pricebook (Quick Add From Pricebook, then Add Item) or a Custom Item, change an existing line's quantity, unit price and description with Save, or remove a line with Delete. Changes can be made only while the work on the work order is still outstanding; once it has been concluded, the lines are locked.
Use it while the work is being prepared and carried out. Once the lines are right, create and send the invoice. Pricing a line away from the pricebook price is a separate action, as is reversing a charge.