Make sure your chocolate is vac sealed, dry ice is around -80 degrees Celsius and will condense any moisture out of the atmosphere onto your chocolate. The length of time it lasts depends on how much you use and how insulating the packaging is. DHL should be able to advise you and should have packages you can use, some companies do top-ups if it is a long journey. From my experience most courier companies don't give a rats about your stuff though. I haven't tried shipping in hot weather but I would imagine that a good cold pack and properly insulated foam package would keep the choc below 25 degrees Celsius for quite a while. Personally I just ship when the weather is conducive, in bubble wrap, I just had a package of choc come in from San Francisco and it was fine, it is winter here but the height of Summer in San Fran. I find packages sit around much longer at the destinations than they do at the departure points. My brother sent me some stuff from Canada when it was Summer here and it sat on my doorstep on the hottest day of the year, 45 degrees Celsius, needless to say chocolate was melted.