Based on my experience with USPS, it should be “Sunday, or whenever we get around to it, if you’re fortunate that we deliver it at all” delivery. “Sunday Delivery” is, however, a little easier to say.
I foresee a lot of future unhappy Amazon customers...
My business mails 100’s of packages daily. 2/3rd USPS, the rest mostly FedEx. The packages sent USPS are delivered much faster than packages sent FedEx (or UPS). Even parcels sent to far away zones USPS get delivered in an average of 2.5 days. Items sent to far away zones FedEx or UPS take 5 days (for FedEx) and 6 days (for UPS). The packages delivered in 2-3 days USPS are sent either 1st class Parcel or Priority Mail (never the slow Parcel Post) and cost less to send than the slower FedEx or UPS.