What heat actually does to a closed truck
A sealed moving truck sitting in a driveway or parked between stops can climb well past the outside temperature within an hour. On a hundred and ten degree day, that is a serious problem for anything sensitive to heat.
This matters most for the middle of the day, so items that need protection should be loaded last and unloaded first, spending the least possible time inside a closed trailer.
Items to carry in your own car
Candles, cosmetics, vinyl records, and anything wax or gel based should ride in your own air conditioned car rather than the truck. Heat can melt or warp these items in a way that is not always obvious until you unpack weeks later.
Houseplants also do far better in a car with the air conditioning running than in the back of a truck, where they can wilt or scorch within the hour.
Electronics need extra care
Televisions, computers, and game consoles should be packed in their original boxes when possible, with cables labeled and bagged separately. Extreme heat can affect internal components, so these items are good candidates to load last and unload first.
If you are packing electronics yourself, avoid leaving them in direct sunlight even briefly while waiting to be loaded, since a hot dashboard or driveway can heat a device faster than you would expect.
Packing materials that hold up
Cheap cardboard boxes soften in heat and humidity and can collapse when stacked in a hot truck. Sturdier moving boxes, especially for anything heavy like books or dishes, are worth the extra cost during summer months.
Packing tape can also soften and lose its grip in extreme heat, so double taping seams on boxes headed into a hot truck is a smart habit.
Scheduling around the worst of the day
If you have any flexibility, an early morning start beats an afternoon move every time in summer. Loading before ten in the morning means the crew and your belongings avoid the hottest part of the day entirely.
Keep water available for the crew and for yourself throughout the day. A hydrated crew moves at a steadier pace, and a steady pace means fewer mistakes.