You don't need that. When you made the purchase a print out of the online invoice will do. Just print that out. That's good enough.
Also I don't think it's legal for customs to fine you because of what the sender did, unless they have proof you asked them to do this.
The sender will get in trouble with his government and postal authority for that. You can't be responsible for the actions of someone else, who is beyond your control.
Well I really hope thats the case because from experience they always under declare. The only place that actually declares the proper value in the past was DX which ironically always gets held up at customs.