In this tutorial I go over what you can do to try to prevent your balloons from popping in direct sunlight.
Inflate and deflate a couple of times before inflating to the size you want
Double stuff with the same colour
Tie the necks closer to the end of the neck, rather than the top of the neck. I know this makes it harder to tie cluster to cluster. But it gives so much more expansion room.
Make sure your TS and CS cover sunlight and heat. Also go over it again with your customer. So they are 100% sure on their decision of having it in direct sunlight.
If the customer is having custom double stuffed colours. Make sure you chat to them about the possibility of the outside balloon popping and leaving the inside colour showing.