I do have good luck with cool packs as long as express service is used. Issue with them is they don't usually last more than 18 or so hours and they add a lot of weight to the box. I bought MAHOOSIVE ones that last quite awhile, but add nearly a pound to the shipping weight. In general, if express is used, fish seem to do really well in a well insulated box with extra packing materials.
And danio tinwini are awesome! I had them in last year and they didn't sell well at all, which shocked me as htey are an amazing little fish. I would be willing to try them again. Will definitely get more dario species, definitely chillis, I am currently breeding longfinned white clouds but would absolutely be interested in getting the vietnamese as well.
Great ideas folks!
I am out of town until Sunday, but will certainly be checking in to see what else people suggested! I get my lists on Mondays and order on Wednesday. I am stoked!