I accidently put a 1st & a 2nd class envelope in the Ist Class slot. Just beat the postman who explained they would be separated later. I can't say I believed him (about 1 out of a hundred).
Think it's either prioritisation or judging from 2nd Class postcards, the 2nd class are the brushings-up of the floor at the end of the week.
I prefer a single-price service. Pricewise, in competition with the internet and telephone, give it its proper name "3rd Class".
After all, deliveries are mainly junk mail - diligently re-cycled by us mugs.
SIQ.