I believe I understand. It is a bit tricky to post items to eBay US using our lister form when you are located in Canada, however, you can do so as long as you are using flat rate shipping.
To accomplish this, instead of entering a zip code, check the radial button next to "I Will Enter the Location:" and then type your location into the adjacent field.
Then, when you are specifying your shipping options, under domestic shipping, use the generic shipping options like "Standard Flat Rate Shipping Service" and "Expedited Flat Rate Shipping Service" to list your shipping costs to the US.
Under international shipping, use at least one of the generaic flat rate shipping options to be used for service to Canada. If you are also offering shipping to other countries, add additional generic flat rate shipping services to accomodate shipments to the other countries/continents you are willing to ship to.
For further assistance with this matter, I recommend filing a support case using the appropriate link on our help page:http://www.auctiva.com/free_auction_software_help/help.aspx