I'd heard on the radio that some SDA communities had Sunday delivery rather than Saturday. At least one of those lost Sunday delivery a couple years ago. There may be a couple holdouts.
The computer-doesn't-recognize-Sunday-as-a-workday thing seems like BS. The USPS will delivery packages on Sunday if you pay them enough.
The SDA/Saturday delivery relationship has some other ramifications. It's hard to work for USPS if you observe the Sabbath on Saturday. Eliminating Saturday delivery might make USPS an easier employer for SDA members to work for.
"Seventh-day Adventist Church Associate Counsel Todd McFarland ... [said] "For decades the USPS has been the single most troublesome employer for those seeking Sabbath workplace accommodation. Halting Saturday delivery will not only prevent many future Sabbath observance conflicts for Adventists employed by the post office, but will help resolve current situations in which mail carrier-church members are experiencing discrimination”".
A Jewish take on mail on Saturday:
A bit more about Sunday delivery from a Sunday-Sabbath site:
Looks like the remaining Sunday deliveries are done by private post offices (contracting to USPS).