When you calculate it lets you choose a shipping service ... for anything bulky use parcel post .. its cheapest .. if they aren't too many states away often priority mail in a regular box from home is about the same price as parcel post .. sounds like you were using express or priority mail .. or shipping from Australia ... whats your zip-code?
So when you go to the page it asks your zip and the buyers zip ... it asks the weight of the package .. for this example I used 5 lbs 0 ounces ... it asks the package type ... you would be using large box ... it asks box dimensions I used 12x12x12 the it offers a bunch of service options ... for items over 13 ounces parcel post is cheapest to ship 5 lbs from NM to KY parcel post is $10.82 .. Media mail ... is special and only for Books, Cd's and DVD's ...They also have a page where you can calculate postage, print shipping label and have the postman pick up the package at you house ...