Border/customs tariffs from American websites ruins any cost savings (like paying $20 in fees for a shipping a $40 item [1]). That is the root problem, given the geographic nearity.
http://www.ups.com/content/ca/en/shipping/cost/zones/customs...
(I use Ship Happens, but there are many more)
There's also an option somewhere to restrict search results to only items that can be shipped to the chosen country. You'll be disappointed by the reduced selection though. In this case, I suggest Amazon UK who has reasonable shipping to Australia.
priceusa.com.au places the order for you and is closer to a personal shopper service (at a premium)
shipitto.com seems to be a mail redirector but has silly postal costs.
myus.com is similar to shipitto.com
I can search for something like "Vacuum Filter" and receive results from 26 departments including Baby Products. So say I go to Home & Outdoor > Vacuums & Floor Care › Vacuum Accessories › Vacuum Filters › Upright Filters. Now I want to search by brand but there's nothing available for my model when I filter the search, however I can buy a Dyson filter under the "Hoover" sorting. Then half the listings are from 3rd parties who only include a lone 120x120 pixel image, an incorrect product description (or none at all), and with 0 product reviews. Then I give up on logical searching, type in the product code and the only listing was in the Automotive department after all.
Sorry to go on a rant but I find the shopping experience on Amazon to be incredibly frustrating. I usually end up using Google or another site and even when I do find the Amazon listing I'm hesitant to buy because of the sketchiness of the listing.
Picking a department is always annoying especially when there are two or more it could easily be in, but overall I like the search options. For example I have wide feed so I always need 4E shoes and Amazon can filter out all of the shoes which don't have 4Es.
A "20 minute interstate drive" is supposed to be a lot?
People commute for longer everyday just to get to work, and you're talking about shopping...