Mystic, the only 'restriction' I pay attention to or discuss with my customers, is that in the UK, if the value of the package goes over 15 pounds (I think that's the limit) - the customer ends up paying an import fee. Sometimes I've had customers order, say, $40 worth of vintage jewels - and I ship in two separate packages, so the value of each is about $20 USD. Sometimes that won't be possbile with jewelry of course - it's just something to keep in mind.
I'm always truthful on customs forms - as others have said, it's totally not worth lying to Customs! But I'm also careful about what words I use. Earrings for example, could be "craft earrings" or "fashion accessory" - one doesn't have to say "pearl earrings", which might be more attractive to a thief.