Phil means the overall length of three edges of the box. As he said in the first post where it came up:

x+y+z

That's apparently how they charge at the post office.

Sent from my GT-N7100 using Thaivisa Connect Thailand mobile app

I have used Un-Accompanied Baggage many times, and it always goes on my same flight. (risk it may not)

This is no good going to Thailand though, as you have to waste a day in customs bribing and clearing it but great outbound.

Normally $3-4 kg

Much cheaper than excess baggage.