I've sent and received dozens of packages to/from the US and had like maybe 2/3 times where there were customs fees.
One time I bought a DH jeans from an american...he put the real value on the package (-_-') ...when it arrived I had to pay like 200€ of taxes (above a certain amount declared on the customs form, it's an obligation to tax it, under, it's not...so they pick random packages to tax)...refused to pay..they sent back the package (free!! i thought they were going to ask the sender to pay for the return but no). So when the guy got it back, I told him to put a low value and send it again, received it with no trouble this time.
Everytime I send packages there i put a low value and never had problems and no one has ever complained to me they had to pay anything once received. I've sent parcels to the entire world, china, japan, australia, thailand....
Another thing is that some companies (ups, fedex...) have included "customs control" and they tax almost every package that go across their hands...
Online store, often for "legal" obligations don't want to put lower value when they send a package abroad...that why you have more risks to get caught...
Not putting any bill in the package won't help either because they will hold the package, send you a letter asking you to send a bill to prove the amount put on the customs form.
Only time it happenned to me, french customs didn't send anything and had to make many phone calls and at the end managed to be connected to the customs service that manage parcels at the airport and stuff and they found my package...anyway, 2 days after sending them the documents, I got the parcel...
