RE: Collector's Cache Shipping Options
I've used Collector's Cache a lot. They were my main shop when I lived in the US for English-language products. My orders from there were always reasonably fast and well packaged. This was only for domestic purchases, though; I don't have any experience with them shipping something internationally, and mostly with booster boxes or promo products, but I'm guess you're only buying a few singles or so?
The $6 sounds super cheap for a shipment all the way to Oz. "First Class Mail" is basically the standard USPS shipping option for envelopes and small packages, with price based on weight and location. This will probably take a couple weeks to get there.
Priority Mail is also USPS, but will get there faster, and they'll always be the same price based on the size of the box ("flat rate" and all). Generally speaking, flat-rate shipping options are best when you're buying heavier things, since you pay the same price for a heavier weight as opposed to the First Class that prizes out based on weight. If you're just buying a few singles, this is probably overkill, but would probably be worth it for full booster boxes.
I would also make sure to double-check the customs limits for your country to make sure you won't get slapped with customs fees when the package arrives.