Nope. It’s not like that. It seems I must change the URL all the time to be able to add the items. So http://www.edinventa.com/?fill_cart=1×491 does NOT work anymore because I used it once.
So to add the products I must to change the URL to any other like: http://www.edinventa.com/blabla1/?fill_cart=1×491
Weird. Isn’t it?