If a package gets intercepted by authorities and they want to try to get some arrests out of it they usually will pass it back to the postal/delivery company and monitor the address and people related to it. Or they'll pose as the delivery (wo)man themselves. They most likely wouldn't send you a request to get to the post office because that way they have to be on standby until you show up. Not efficient, you can take a couple of days to show up. And you can choose your time, they like to do that.

I notice that when they want to bring down drug lines or production facilities here in the Netherlands they tend to observe and wiretap all people connected for many months, even years sometimes. If they have the impression that they might uncover more by observing they'll let it go on. Also with big drug lines they discover by chance through their routine process, they'll let the shipments through as if nobody knew about it. And then after a couple of successful shipments they all monitored and documented they strike.
IMO it is an indication how dangerous they really think the common illegal drugs are to public health, not that bad that it should be stopped ASAP. No just let it go on for a year, build a case and then take it down. And if some bad dangers pills make it to the market they often catch a number of people that sold and made them within weeks. Although I haven't heard them catching someone producing these crappy 4FMP/FA pills that came by several times recent years with some LODs.
Hehe, just did a search on 4-fluoramfetamine substance and it only became illegal in my country as of May 25th, 2017. Can you believe this shit, people (if we are to believe the media) died of this stuff, it had been around since 2005, but it took them 12 years to prohibit. All the while, the way less dangerous 3,4 MDMA was illegal. How about that for causing dangers to public health.
Acid or mescaline are prohibited but you can get some really dangerous new RC chemicals in bulk if you want all legal. Amazing in an unsatisfactory way.