$51 is very cheap for a set of these. If you find another cheap set which is interstate, and the seller is willing to pack them up, try these guys:
www.e-go.com.au I've used them for a few different items (set of wheels, big box of parts etc) and they have been quite reasonable in price with excellent service. The wheels were in the one box and they cost me $31.39 from Melb to Canberra 18 months ago (net weight 22kg; cubic weight 36kg).
The seller packs the item and gives you parcel dimensions and weight. You plug in a pickup address and pickup day/time, your delivery address, and the parcel details. You pay online, they send you a consignment note for the parcel via email attachment. Email that to the seller. You can even save around 25% for depot to depot delivery.
May be an option for you.
Cheers
Jamie