USPS packages do constantly move through the USPS sorting facilities on Sundays and everyday 24 hours per day 365 days per week.
It's only the local post offices that close on Sunday and do not sort mail or deliver mail on Sunday but the mail is constantly being sorted and being delivered and traveling on truck or airplane all the time even on Sundays through the USPS.
That is to make sure you get your mail and packages on time through the USPS.
U. S. Mail through the USPS is always on the move either in trucks, planes, or in processing centers everyday of the week including holidays and Sundays.