The interesting thing with both FedEx and UPS is that they also guarantee delivery times for regular "ground" shipments, which are fairly prone to delays. The exceptions are circumstances out of their control, such as weather or recipient issues. And they suspend during a couple holiday windows. But most shippers either don't realize there are guarantees or don't have the means to efficiently track them.
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The interesting thing with both FedEx and UPS is that they also guarantee delivery times for regular "ground" shipments, which are fairly prone to delays. The exceptions are circumstances out of their control, such as weather or recipient issues. And they suspend during a couple holiday windows. But most shippers either don't realize there are guarantees or don't have the means to efficiently track them.