I don't have any official statistics to look at, and this largely depends on your country, the contents of your parcels, the shipping service, the declaration and how it was packed.
With that being said, I can give you a guess. I'd say that 95% get through without any questions. Maybe 5% get pulled for further inspection and questioning, 50% out of which get released, while 40% of these can pay their way out and only the remaining 10%, which is 0.5% of all parcels are confiscated and destroyed.
added: 6/26/2023