FedEx SmartPost and UPS SurePost products are a reduced cost way of using FedEx or UPS to send packages. It is an up front agreement between the shipper (Amazon in this case) and the shipping company (FedEx or UPS) that the package can be delivered to the local post office for final delivery instead of being delivered by the shipping company. The shipper chooses whether or not to use SmartPost or SurePost. They can choose regular shipping where the shipping company always does the final delivery ... but the cost is usually higher. FedEx and UPS save money by not needing to visit every location to deliver packages, but the scheme can delay packages (depending on when FedEx and UPS deliver to USPS and when USPS does their sort to the local carrier's route). In a best case scenario FedEx or UPS gets the package to USPS before the USPS sort and the package is delivered the same day USPS gets it. But if that particular post office does not receive the package in time for the sort the package will be delivered later. Saving money is how Amazon and other shippers keep their prices down. If shippers want guaranteed delivery within two days they should pay for two day shipping that does not include SmartPost/SurePost. But such guaranteed shipping is not cost effective if "normal" delivery through SmartPost/SurePost or standard ground service is "two days" or less. It is a gamble to "hope" that a non-guaranteed service will be delivered on time. A gamble Amazon usually wins. But should a package be delayed they can fall back on their actual terms of service - the legal guarantee - and either refund two day delivery (if paid for on an individual order) or extend Amazon Prime within the limits of their terms of service. The TOS seems fairly clear ... both in what they are promising and the remedy for when the promise isn't met.