I receive many items from USA through USPS/Parcelforce without any problems and it is very unlikely the parcel has been opened from America. As you say the reason will be that the seller did not fully declare the contents. I always declare as a either a watch bracelet - $15-$20 or costume jewelry and always tick the gift option. Just telll them it is an early xmas gift and make up what is inside as I doubt they will open and check, they never have with me yet anyway.