Which of these services guarantees delivery on Sundays and Holidays for an additional fee?

The service that guarantees delivery on Sundays and holidays for an additional fee is Priority Mail Express. This service is designed for time-sensitive deliveries and is the only USPS mailing service that offers such a guarantee. It ensures that items sent with this service will arrive on the specified date and time, including Sundays and holidays. This is particularly beneficial for customers who need their parcels delivered urgently or on a specific schedule, making it a preferred option for those who require reliable and expedited shipping.

The other services mentioned do not provide this guarantee. While Priority Mail offers two to three day delivery and may include weekend delivery in select areas without a specific guarantee for holidays, it does not carry the same assurance as Priority Mail Express. Standard Rate Mail and First Class Mail also do not guarantee Saturday or holiday delivery. Thus, Priority Mail Express stands out for its unique ability to provide guaranteed delivery at all times, making it the correct choice in this scenario.