The randomness of freight costings stems from unlike Oz , the US have around 26 separate companies registered to ship domestic mail items. Then you have general carriers and international forwarders and they all have different pricing structures all based on how they can utilise their network and take advantage of any rebates or discounts they can glean from another carrier who base their costings on how they can tie in with another schedule. This is why the different times and prices for the same items and why your item seems to go back and forwards to a central distribution centre before heading off shore. Confused? Good, thats what they want to do to you.
Whos the fastest and cheapest? Good question and I wish I had a answer it deserves but I dont.