Customs value and insurance are indeed unrelated. The seller can put on the customs form whatever he wants. What is written on the customs forms is not the responsibility of the postal services. They just have to make sure you put a form on the shipment.
Insurance is unrelated to that and the amount the shipment is insured for is not visible to the customs officers of the receiving country. So you can declare 20$ to customs and insure the shipment for 800€. No problem here in Germany, done it many times.
Armin
This guy obviously isn't very savy, because he asks the shipping company, and of course they're not going to tell him how to get around it. Here's his reply:
"I asked the shipping business again.
It's like I had said: The tariff provides for the insured value!"
So they're either going to be declared at a high value and I risk paying who knows what in fees, or I risk it and not get it insured. I'm fairly certain most stuff I buy internationally is not insured, I know I don't ask it to be and the shipping is usually low. My only reserve is I've never bought something this big internationally. Honestly I only wanted one of these posters, but I'm buying all four since shipping is so high it just seemed stupid not to and save. So to insure or not to insure?... that is the question.