Do truck bed covers protect cargo from getting too hot?

I have a Roll-n-Lock bed cover on my Tacoma. I love it and know the benefits of saving gas and protecting cargo from rain. I am moving from the DC area to TX and the movers will not move anything flammable (ie. paint, cleaning supplies, candles, lighters, etc) so I will have to move those items myself in the back of my truck. As I'm driving the long distance with the sun beating down as a get farther south by the end of May, how safe are those type of items in the back of my covered truck bed? I'm uming it should be okay, but would like further input to be sure.