shipping charge question

I am curious as to whether this can happen:
I am having a problem with the canada post module in such that the shipping charges that are being calculated within the module are not what they should be (usually way less than what the post office is telling they should be). So... is there a module or way that I can have the customers choose their items - then and then have an email sent to me with their shopping cart and location - then me being able to calculate the "correct shipping cost", let them know and they add the shipping total into the cart and finish the checkout process? I am also finding that if I use flat rate I am either losing money or charging way to much.

Thanks for any help suggestions

DJ
http://www.shadoblixfarm.com