Understanding the doublings
If you seek to please 90% of your potential customers, all you need to do is the usual thing.
To please half the remaining potential market, you're going to need to work at least twice as hard.
And to please the next half, twice as hard again. It's Zeno's paradox, an endless road to getting to the end.
So, a letter with a stamp gets you on time deliverability 90% of the time.