That's why I think it's always better to include the shipping cost for the areas you ship, Have 3 shipping areas one for Canada second USA Third over seas and near
the very bottom select will ship worldwide.
for most items that are very close in weight put the average shipping cost, loose on some and gain on another.
for items that are like very big or heavy use the Built in Shipping Calculator, if setup properly it works very well, you can even use it for all your Listing's and just mention the average shipping cost in your listings if you wish, since many buyers don't want to bother to type in there zip code.
By doing the above you know for sure your listings will show up on all the sites, also make sure you have all the country's selected in your Powersellers prefences
hope this helps you
:)
T.J.