You can add custom items to bookings and add a custom price to the added item. E.g. champagne brunch on a day tour or wine bottle to a accommodation invoice.
Each new line items is added to the base price of the booking. You can add taxes and discounts manually to each of the line items.
To add a line item to a booking the first thing to do, is to find the booking and open it, and click on the INVOICES tab.
When in the booking select the "add a custom line item" button. Add the name, quantity, unit price, discount, tax (if applicable).
You can send an updated ticket to the customer by selecting the e-mail button in the grey right hand bar.
The final invoice will look something like this with the extra line.