Customs authorities use the declared value to determine whether the goods shipped comply with their customs clearance regulations as second-hand personal effects and whether the declared value does not suggest that the articles are new and imported for resale, or for any other reason other than for personal use.
Only personal property will be cleared through customs. Therefore, your declared value must reflect the estimated resale value of the contents of your luggage or parcel and not the price of new items. For example, a toothbrush would have a resale value of a few euros. For your information, a suitcase of used clothing will most likely have a resale value of less than €100/$150.
However, the customs declared value has nothing to do with the insurance.
If your shipment is found not to comply with duty-free entry regulations, it will be the responsibility of the consignee to complete all customs clearance procedures and pay any duties, taxes, or fees. If you are sending new or commercial items, payment of import duties and taxes may be required from you.