Why do Alibaba sellers put random stuff in the final contract?

So for the final order through trade assurance, my suppliers put random items into the order (so not the items that we agreed to order), such that the sum is the correct amount that we agreed to. So I am still paying the correct price. Why not just put the actual items? Is there a reason for this? and is there any potential complications?

Also, what do sellers mean regarding a low declaration such that the import taxes are cheaper. They ask me how much I want to declare. How would I know how much lower than the actual price I can declare without something going wrong? Is there somewhere I can find out more about declaring a shipment lower than it actually is to decrease import taxes? Because I’m very clueless.

Thank you in advance for any help