Although filling an entire trunk full of peanut butter, let’s say 500kg worth (assume a 450 litre trunk and density of 1.1 kg/l according to Google), then adding all that weight over the rear axle would affect the handling and balance of the car, potentially making it dangerous and therefore illegal to drive.
So, it’s not actually that clear cut that it wouldn’t be illegal!
My mom's from way out in the country. The first and only time she was stopped on suspicion of drunk driving was when she had an entire butchered pig from her parents in the back of her station car making it swerve.
And that was less than 1/4 the weight of that peanut butter collection 😄
Most SUVs cannot handle the weight, let alone a car. A typical SUV will allow for 200-300 lbs in the trunk. At least according to the GVWR and with 2 passengers.
I got a bunch of free Coca-Cola once, and some dude at taco bell overheard me say something about "all the coke I have in my trunk". He then tried to sell me some of his coke.