The free books which say $0.00 on the product page are completely free, regardless of whether or not you use the Whispernet to download them.
The usual $2 Whispernet fee is included in the regular buying price of the book, as stated on the product page, which is why Canadian prices for non-free books always look exactly that much higher than the corresponding US price. It's paid regardless of your download method and whether or not you have an actual Kindle to take advantage of the wireless delivery in the first place.
You won't be given any separate charges beyond the website page's product price, besides applicable taxes.
To get full access to the US Kindle store, you only need to set your Kindle's country to the US, with a US billing address. There's a workaround with using Amazon.com gift certificates to pay for everything, so that they never actually check, much less charge, any credit card you use.
In these cases, I advise avoiding using Whispernet for purchases, and instead downloading them via computer, because it just seems like it's easier for Amazon to catch you if you use the wireless all the time from the "wrong" region.
Mind you, from time to time, some stuff shows up that's only available to Canadians, such as this
currently free police procedural mystery/thriller by Peter Robinson.
You might want to try downloading one of the Kindle for PC/Mac apps and "purchasing" a few of the promotional freebies; see how much you like how the Kindle shopping/reading experience works, and if it turns out you do, then you'll have a nice stock of free books to load up on your Kindle if you decide to get one.