What is the temperature in the storage compartment on board of a cargo aircraft? I'm ordering a temperature-sensitive package by airmail and wouldn't want it to freeze. There are many questions related to temperature of the luggage compartment on a passenger airplane but I didn't find anything about cargo-only aircrats.
Many things including bottles containing liquids break when the temperature drops below zero, so the cargo hold is always maintained above freezing; 7°C is typical (source). This applies to both pure cargo aircraft and mixed passenger/cargo airplanes.
That said, it's harder to control for (say) cargo sitting around on tarmac while waiting to be loaded on the plane. Your cargo shipper can likely arrange temperature-controlled shipping for you, but at a steep extra cost, and this is usually meant for perishables where the main danger is heat, not cold. But unless you're shipping something to/from Siberia, you'll likely be fine.
Temperature conditions in cargo planes are no different than passenger planes. Interior spaces are heated to some degree, as outside temperatures at 35,000 feet are too cold and could damage all kinds of cargo if allowed to stay that low. When they are carrying live cargo then that cargo is placed in a section of the aircraft that is heated to a warmer temperature to assure that cargo arrives alive.
One more thing you should be aware of, pilots are given the cargo manifest by the load master before the flight. Sometimes, the cargo contains temperature sensitive loads and thus the crew can adjust the temperature of the cargo hold.
This holds true for cargo and passenger aircraft.
For example, if transporting live animals the temperature is increased above the "normal" range.
If your item is suitably marked and you have notified the shipper of the same, they will ensure that it is secure during the flight. However, as mentioned by @jpatokal - your main worry would be the time it spends on the ground, being driven to-from the cargo hold, etc rather than the flight on the aircraft.
Check with your shipper to see what they can do to prevent any damage to your item.