You can add extra hand or hold luggage to your booking by visiting your airlines website and clicking on the manage booking section.
You'll be able to select any additional bags and pay the airline directly. Login details vary by airline but in most cases you’ll need your Airline Booking Reference which you can find in the Flight Voucher on your booking confirmation paperwork or by logging into My Bookings. See more information on how to find your airline booking reference here.
Just a heads up - any bags you add directly via the airlines website won't show up on your On the Beach confirmation paperwork or in My Bookings - don't worry, though - the airline will have everything they need on your flight booking.