Sadly, chilling enroute can do some serious short term damage to baby chicks. Shipping during a cold snap is almost guaranteed to produce dead chicks, and those that don't die, have been dealt a set-back if they were chilled. Usually it's the chicks on the perimeter of the shipping carton that both catch the brunt of the chill and end up shielding the others with their bodies.
Sometimes, a little nutritional boost can make a difference. A lot of us administer Pountry Nutri-drench to new chicks upon arrival and then a diluted dose in their water for their first week.
Type of heat can also make a difference when you're dealing with chicks that may have suffered chilling. I've found that these chicks crave extra heat, and direct contact heat is what satisfies their need the most. If you can rig you heat system so they can make contact with a warm surface, they seem to fare better.
I don't know if the Brinsea lends itself to this, but a heating pad would.