They are shipped in tiny bags because they don't need much water (air Breathers) this actually stops them from eating there tails, and if shipped correctly they should only be in them for 24 hours max, thats from a good shipping dealer I'm sure if they have fin rot then they were put in the bags with it it would not happen over night OK exceptions is shipping illegal ems that takes longer and the fish is put under more stress but if healthy in the first place should survive 4 days in them small bags with a heat pack this time of year......... 
