I am in Sydney, Australia and this is probably the hottest continent on Earth. It's also very larg. I send chocolate around Australia and indeed to New Zealand and other coutries and this is how I do it.
This follows ask-maki's suggestion and it does work very well. I get VERY few failures and if I do I just replace.
At markets I use the same idea but one really important concept is to keep the cold air from getting out and new hot air getting in. To achieve that I pop the chocolate in the foil bag and then I heat seal it on the stand. Then the air simply can't transverse.
I can't actually buy these bags - at least not cheaply. So I buy rolls of metalized of the material (metalized bubble wrap) that I use for shipping and cut bags out of that and make the bags myself at home before going to the markets. Then I just have to heat-seal the opening on-site.
I DO keep all of my chocolate in polystyrene boxes on my stall to keep them cool. And I take these nice cold bags to seal for Customers.
Oh - forgot to say. I keep my chocolate as cold as I reasonably can and load my van just as I leave for the day. Temperture is always around 15-18C.
Hope this helps - taking care of chocolates and Customers at farmers markets becomes a real artr and I am now pretty good at it!
Captain Chocolate, Sydney, Australia