The USPS usually keeps the packages inside & warm,
I did an experiment that proved that is not true..
about 4 or 5 years ago I did a survey on shipments of photo paper for Kodak, my employer at the time.
The goal was to test the temperature inside trucks going across the country, in the winter, but USPS was also part of the chain.
I put a temperature sensor on a pallett of rolls of photo paper, to record the temp on the whole trip..
(the trucks going out were *not* USPS.they were a private trucking line..we wanted to test the temps inside those trucks,
they were supposed to be heated to a certain minimum temp..if they werent, it could effect the paper emulsion)
The temp sensor recorded the temp every minute, and had the capability to record for a week.
The pallett was loaded into a truck in Rochester, NY, and headed out to Colorado.
When the truck arrived in Colorado, the temp sensor was removed, still operating, and put in the US mail, to ship back to me in Rochester.
I knew the time the sensor was placed in the mail..all dates and times were recorded as part of this test.
This was done multiple times, probably 20 to 30 times, over the course of several months..
because I knew the time the sensor was dropped in a mailbox, to travel via USPS back to me, I could track the temps
while it was in the care of USPS..result? really really cold! most of the time, USPS packages are exposed to the cold in the winter.
they do *not* keep packages inside and warm, for the most part..I dont think their larger trucks are heated at all.
Scot