That's because the Postal Service started delivering small parcels in about February 2015, just after they eliminated overnight service for 1st class letters. That was a response to the concerns that people would have to wait longer to get their meds by mail. Many pharmacies were refilling by mail and mailing locally, so it would be delivered next day, and customer surveys had shown strong support for that, so now there are carriers out delivering those on Sunday. It was cheaper than maintaining the equipment and staffing to continue overnight local mail overall.
Yes, trust the government. They’d never lie to you.