According to the DMM (Domestic Mail Manual). Section 113
4.2 Express Mail Next Day Delivery
4.2.1 Availablity
Express Mail Next Day Delivery is available at designated USPS facilities, designated Express Mail collection boxes, or through Carrier Pickup or Pickup on Demand service, for overnight service to designated 3-digit or 5-digit destination ZIP Code delivery areas, facilities, or locations (Post Office to Addressee Service). For an additional option, see 4.2.5, Hold for Pickup.
4.2.2 Where Not Available
Next Day Delivery may not be available at or between all Post Offices or at all times of deposit. An Express Mail Next Day Delivery directory, showing detailed local information about Express Mail Next Day Delivery, is available at Post Offices.
4.2.3 Acceptance
Express Mail Next Day Delivery items must be presented no later than the times authorized by the postmaster. Express Mail Next Day Delivery items mailed after the time authorized by the postmaster are accepted for delivery on the second day after mailing, subject to the standards for this service.
4.2.4 Delivery Times
Items are delivered by noon or 3 p.m. on the next day. If delivery is not made, the addressee is notified; a reminder notice is also left on the third day. A second delivery is attempted only upon customer request.
4.2.5 Hold for Pickup
Under Hold for Pickup service, items presented under 4.2.1 are available for claim by the addressee at the destination facility by 10 a.m., 12 p.m., or 3 p.m. of the next day the destination office is open for retail business.
4.2.6 Refunds
For Express Mail Next Day Delivery, the USPS refunds the postage for an item not available for customer pickup at destination or for which delivery to the addressee was not attempted, subject to the standards for this service, unless the delay was caused by one of the situations in 114.2.0, Postage Refunds.
4.3 Express Mail Second Day Delivery
4.3.1 Availability
Express Mail Second Day Delivery is available to any 3-digit or 5-digit ZIP Code destination not listed in the Next Day Delivery directory mentioned in 4.2.2 (Post Office to Addressee Service). Items are delivered to the addressee by noon or 3 p.m. on the second delivery day. If delivery is not made, the addressee is notified, a second notice is left on the third day, and a second delivery is attempted upon customer request. For an additional option, see 4.3.4, Hold for Pickup.
4.3.2 Acceptance
Express Mail Second Day Delivery is accepted at designated USPS facilities, at Express Mail collection boxes, and through Pickup on Demand service.
4.3.3 Acceptance Times
Express Mail Second Day Delivery shipments must be presented by 5 p.m., or such later time authorized by the postmaster. For Second Day Delivery, the USPS refunds postage for an item not available for customer pickup or for which delivery was not attempted, subject to the standards for this service, unless the item was:
a. Delayed by strike or work stoppage.
b. Made available for claim, or delivery was attempted within the times specified by the standards for this service, and then the item was delayed because forwarding or return service was provided.
4.3.4 Hold for Pickup
Under Hold for Pickup service, items presented under 4.3.3 are available for pick up by the addressee at the destination facility by 10 a.m., 12 p.m., or 3 p.m. of the second delivery day that the destination office is open for retail business.
4.3.5 Refunds
For Express Mail Second Day Delivery, the USPS refunds the postage for an item not available for customer pickup at destination or for which delivery to the addressee was not attempted, subject to the standards for this service, unless the delay was caused by one of the situations in 114.2.0, Postage Refunds.
So, if delivery did not fulfil the above standards, then you will get a refund. As deerman said, you must fill out a form (and have your receipt from your mailing). Don't know the form number, but I can find out if anyone needs it.