From experience with UPS and USPS, oftentimes, when a package is marked as "Delivered" but the recipient can't find it, it's because USPS didn't actually deliver it.
Also from experience, in situations like these, we've learned it can typically take up to 3 business days until the package is actually delivered.
We know these things can be super frustrating, but if you have found yourself in this situation, we ask that you allow 3 more business days to see how things shake out. You may also want to contact your local postal facility for details.