I think though, that the percentage of USPS first class mail delivered in 1 day (including Saturday) is probably pretty close to 93% since a significant fraction of mail will be within narrow regions (I would guess that the DC–Boston area itself accounts for at least half of first-class mail).
Back in the Netflix DVDs by mail days, it was rare that it took more than a day for the DVD to get to or from the Netflix warehouse (when they started shuttering warehouses in the last few years of service, that went up to two days each way for a DVD from Chicago to Columbus, Ohio).
I’d also point out that mailbox pickup in the U.S. varies by the mailbox, but many of the ones near me have late afternoon (3–4p) pickups and if I go to the mailbox by the local post office it’s 5.30p. I can even take it to the downtown Chicago post office for a 9p pickup.
In the UK, 93% of first class mail is delivered next day, including Saturdays, and you can often post things as late as 4pm.
Whereas USPS first class mail is delivered in 1–5 business days. Admittedly, in a much larger country.