This is with Calculated Shipping, right?
There is a small disconnect between Canadian and USPS dimensions. The fix is a little white lie.
First, make sure you are entering all your dimensions in metric. Canada Post went metric in 1974 and everything else is inaccurate. You have to click the Options link and scroll down to find the metric button.
Then go to your dimensions and add one centimeter to any of them.
This makes eBay think your package is just big enough to fit the USPS requirements and your US s&h will start to appear.
You can double check by, immediately after uploading your listing, look at it and enter 90210 as a US zip code (or any other, that one is just memorable). The US rates should appear.
If not, go to Revise and add a centimeter in another dimension. But this probably won't be needed.
