Ask HN: How priority (prioritaire) mails processed?

One of the biggest mysteries in my life is those little blue "Prioritaire" stickers on the mail envelopes.

Let´s imagine, a bunch of mail arrives to the distributor center. Some of them has priority, the others not. In order to get know, which ones have to be delivered with priority, they need to check all mail, one by one, for the little blue sticker. Technically it is simply possible, to create two stack of mails: prio och normal.

But what´s next? Do they intentionally pause delivering the normal ones, just for deliver the prios earlier? I don´t think so. Once the mails are allocated to the mailman, there is no rational reason to NOT deliver them all at the same time.

And if this is right: what do I get for my money when I purchase priority?