Custom's guys aren't particularly dumb these days. If they suspect something is dodgy like a brand new twin plate clutch for 10,000 yen they're well within their rights to open the parcel, examine the contents and find out what the real value is.
It only takes a google search most of the time to find a retail price for most of the items people ship and that's exactly what they do.
If the parcel is declared incorrectly then there are penalties....
Whenever I buy big heavy things I always put the correct value down. So I pay the tax big deal i'd rather pay a little tax then be in trouble with customs....