To add, edit, or remove line items on an invoice:

1. Open the invoice in edit mode (or create a new one)
2. Click "Add Line" or the + button to add a new row
3. Enter description, quantity, unit price, and VAT rate
4. To remove a line, click the trash icon next to it
5. Save the invoice when done

Tip: Use clear, specific descriptions for each line item to avoid customer questions.