I've had parcels opened on occasion. When that happens there's often a tax to pay and always a hanfdling fee (did I ask them to handle anything?

)
It's based on value. Above a certain value (depends on your country) there are taxes to pay. I think thatbelow $30 it usually isn't a problem.
Sometimes you can get away with larger values if the gift box is ticked, but I guess they are more suspicious of that. A safer bet would be to have a lower value for the goods declared on the box. Of course, all that is illegal and I can't recommend to anyone you buy goods from to do that. It would simply be wrong... *cough*.
One might even say it would be naughty.