How to know when food grade plastic has melted? I just received a super hot delivery of soup in a polypropylene container (the plastic symbol on the bottom is a 5). I noticed the lid was very pliable in comparison to the rest of the container to the point I could easily bend it and cause the surface to warp by poking it with my finger. After lifting my finger it would bounce back somewhat, but retain some of that warping. As it cools it's returning to it's original shape and becoming harder to manipulate.

I'm wondering if this implies melting/leeching has occurred and if I should just return the soup to be safe.
Plastic is an amorphous solid. This means that it doesn't have a sharp melting point like water, but it goes through a state where it is softer and softer, until it turns liquid. 

So yes, the plastic was in this state when you touched it. If you want to call this transitional state "melted", then it was melted. Unless you prefer to call it "softened". Everyday language is blurry enough to permit both. 

If you are worried about melted plastic actually mixing throughout the soup, like vinegar would mix if you were to add it, that's not what happened. 

As to "leeching", this is not well defined and not usually covered by food safety rules. Plastics do tend to have overheating restrictions over which they char, but as for "leeching", the only regulated one I know of would be melamine, and I'm pretty sure that's not what your box was made of. So throwing it out or not depends on your personal feelings about leeching.