If I buy NAC as a loose powder in a large bag, as opposed to in tablet or capsule form, will it spoil? I've heard it can spoil or degrade into a harmful by-product.

I take do not age glycine & nac tablets and when I open the lid it smells awful are the off?

But what about exposure to heat during shipping or sitting on the front steps in the sun for a few hours? That is my concern when purchasing bulk supplements. What about freezing winter temperatures. Does that have much effect on supplements?

If you store your powders in wide-mouth, air tight bottles like Nalgene (Polypropylene) bottles of 500 mL/approx. 16 oz or 1000 mL/approx 33 oz, you should be fine. 1 Kg will generally fit into a 1000 mL bottle. For speed use a wide mouth funnel. Be sure to add one moisture removing, 5 gram silica gel packet (paper, not cloth), so the powder won't turn into a brick. Be sure to cap the bottle tight immediately after spooning out the powder for use.
Amazon sells less expensive Nalgene-like bottles. Always turn cap tight. Never had one leak and we have bought dozens of them for both liquids and powders. They also sell the silica gel packets. Read the reviews so you'll get the right packets.