You can add Extra purchases to existing bookings from within the booking, these will be automatically added to the amount owed for the booking and shown on the invoice.
To add an Extra to an existing booking:
View one of your bookings and click on the 'Sales' tab
Click 'add item' and search for one of your products. Or;
Click 'Add custom item' and type in the name and price of the item.
Save the booking