| | Re: shiping plants in 100+ degree heat.
See if you can arrange to ship it to the buyer's office, if they work in one. Most office buildings have indoor mailrooms, so sending a package there ensures that the box is held in a climate-controlled environment.
If that's not an option, USPS allows for boxes to be held at the recipient's local post office for pickup, instead of delivering it to their house. That requires a bit more work on the buyer's end to receive the shipment, but it may be the best way to keep the plants out of direct heat.