Where or not to put correct information on a customs form is a subject for debate among sellers.  Some sellers have no qualms about putting a lower value amount on the customs form to save their customer some fees on arrival.  eBay reports that several international countries have contacted the USPS about the declared value on many customs forms being lower than the actual value on the invoice.  Some countries will being levying fines when the declared value is less than the value on the invoice.  See eBay announcement below.
 
Here are just a few points to consider: 
  • The USPS requires that their customs form not be falsified.  The instructions read " A false or misleading declaration may lead to a fine or to seizure of the item."  When you sign your name on a customs form, you are verifying with your signature that the information you provided is correct and valid.   
  • When you state a lesser value for the item you sold, you might save your customer some money, but you'll be transmitting false information.  If you need to make an insurance claim, and you put in a lower value on the customs form, your reimbursement will be at the value you declared, and you'll lose money. 
  • The back of USPS customs forms have instructions for each line of the customs form, with helpful information such as "Country of Origin" is not where you shipped it from, but where the item was "produced/manufactured or assembled."  So, the sweater you sold may be shipped from the US, but it was manufactured in China. 
  • There is a box to specify what the postal service should do in the case of nondelivery.  If you have the items returned to you, you may be subject to paying return charges.  If you instruct the postal service to "Treat as Abandoned," your shipment will not be returned to you.
If you have questions about the USPS Customs Form, you can call them at 1-800-ASK-USPS® or (800) 275-8777.
 
 
Postal administrations in several international countries have contacted the United States Postal Service about the declaration of value placed on customs forms. They have noted that in many cases, the declared value is understated when compared to the actual invoice or known value of the item, especially for sales conducted over the internet. While this applies to sellers both on and off eBay, the USPS has asked us to remind our sellers to declare the proper value on all customs forms.

The correct value of each item is required on customs forms as it helps determine the proper duties and taxes, and whether they should be applied. Some countries will soon begin taking steps to address this issue, such as levying fines on the addressee when the declared value is determined to be lower than the invoice.

To make sure you're complying with international regulations, please make sure the stated value on your customs declaration is supported by your invoices or other documentation.