mystore.americommerce.com/store/addtocart.aspx?itemId_1=25&qty_1=1&itemurl=http://www.www.externalsite.com/producturl
in this line, mystore.americommerce.com/store/addtocart.aspx?itemId_1=25&qty_1=1 is the true url, and &itemurl=http://www.www.externalsite.com/producturl is just a syntax, so it's using itemurl as an attributing factor the the first part. If you are trying to put a product url, you can't make it add to the cart and go there at same time, as it's all one process. But if you're trying to add the item url AS an item variable itself, then that variable needs to be defined, just as if it were a quantity.
also/or it could be that you have http://www.www.externalsite.com/producturl instead of http://www.externalsite.com/producturl