This is no legal advice. I can't possibly know every jurisdiction. This is just my personal way I always thought about this.
What the agent prints out and attaches to the parcel is the merchant's invoice, this is what's being used for the declaration. And as the name implies, it is the responsibility of the merchant, in this case the agent. It makes no sense to hold you responsible, because under normal circumstances you've never seen this before and have no control over it.
That's why I say, if you have to provide proof of the value to customs, they won't care what the merchant's invoice says and neither should you. When you are building your case, start from a clean sheet, just make sure to not completely forget any items off the list.
Bottom line is, having a unrealistically low declared value might have customs take closer look, potentially resulting in you having to pay more. But it shouldn't result in legal consequences, from everything I heard.
added: 6/26/2023