I recently had a package from HobbyLink Japan intercepted by Customs and I had to prove to Customs the value stated on the customs form was what I had payed for it before they would release it.
Since I had payed with Visa it meant a trip down to my bank and a print out on the banks stationery of my Visa statement showing the transaction then a trip back to Customs to show them the proof.
This has only happened once in literally hundreds of packages I've imported but does high light how Customs can pick up a undervalued package.