Do you just play dumb? Ignore any letter that says your package has been seized?
What happens if you got really unlucky and got caught multiple times, would they bother taking action? I can't really find anywhere that talks about this kind of thing in detail.
Over the years I've got quite a few letters from customs in place of siezed international shipments.
If they open a package containing an unidentified substance, they will send some of the substance off to a laboratory where they'll put it through a GC/MS. Even, e.g. if they are clearly branded pharmaceutical drugs. They have to find out exactly what's in the package. This'll take time - sometimes a lot of time. Once I think it was over a month from the point where the package was seized until i got the Love Letter.
The letters are super polite - "...you were sent a package containing 150 1mg alprazolam tablets", etc. and customs go out of their way to make clear that they are not imputing explicit involvement - there is a note in the letter about contesting their declaration in person at some date in the future.
As has been stated elsewhere, the hypothetical "address flagging" implies the existence of some bureaucratic scaffolding that nobody is sure actually exists. It's probably sensible to assume that it does.
The main point is that you don't really get an opportunity to do something - if you get a Love Letter, customs are saying "you are never going to receive that package, and we're not going to do anything to you". If something like a controlled delivery takes place, it will not involve one of those letters.
An entirely anecdotal diversion: when I was younger I ordered absolute fucktons of drugs from all over the world to the UK. Let's just say that my, uh, "opsec" was bush league - I was putting no effort into trying to get away with things, and it's embarrassing how much I actually did get away with. Almost nothing ever got seized. The one or two times it did, I continued to order, and nothing happened.
Once I had half a kilogram of Diazepam powder sent from China, declared as "vitamins" on the customs form, and it was opened by customs. There was a Material Safety Data Sheet in the package for D-xylose - sugar, pretty much. Customs just taped the package closed and sent it on to me. Because they were convinced enough not to test it.
You have to do something pretty nuts for the government to go after you because of some drugs you're receiving, over here.