There's no reasonable way to tell if something is solicited or not, mail is not duplex, and it's not on demand. Most mail I get that matters was not explicitly requested to begin with.

As well, most ads I get in the mail look like normal mail anyway, and I don't really want the government opening my mail to check if it's spam or not.

I'd much prefer a way to opt out of receiving advertisements wholesale on a per-address basis (not per-person, mind you). While you can "opt out" of receiving ads, it's borderline impossible without giving PID to advertisers!