Not had one opened, they usually use the green customs form as a guide.
Likelihood of your parcel getting stopped is variable and depends on
1) whether the value declared is high
2) how busy they are......
Interesting (and sad to reflect on this) but after 9/11 they didn't do any of this, and I remember not paying for anything for at least a year. They were all too busy checking parcels from Iran, Africa etc.
For expensive items use FedEx or UPS Costs a bit more, but a lot faster, and I don't recall them ever asking me for money.
Steve