Let me just throw a few things out here since the holidays are quickly approaching. Pretty much all couriers are hiring for seasonal help right now, USPS, FedEx, UPS, DHL, and even Amazon.
Things are going to effect package delivery, snow and ice probably for the most part. Temp hires who aren't trained very well - misdeliver sometimes, and we all make mistakes even the regular people.
A few days ago on our local news during some heavy rain, a whole USPS transport trailer crashed and spilled it's entire load all over a major highway here. They had to shut down the entire highway to collect it, no doubt all of it got soaked, some was probably ran over and damaged or destroyed. It happens, but not often. 2 weeks ago UPS delivered a package to my house at 11:30pm, we were asleep but the dogs barking woke us up and sure enough....we saw the driver was hoping back in his truck after leaving a package on our porch. 11:30 at night is still within "the same day" it said it would be delivered. A month ago Amazon delivered one of our packages, 7 doors down. Thankfully our neighbor read the address correctly, and brought it down to us.
As far as specific USPS, if you ship or expect a package...pay the extra for tracking. It's not much extra if not already included in the postage price. Another thing, Priority Mail 1,2 or 3 day and First Class has NO guarantee on delivery date, it's just an estimate but it does include tracking and is insured automatically for $100. If you need anything above that, you can insure for more specifically at an added cost. Also, if you need or want it faster or a guaranteed arrival date/time...use Express Mail. It is Guaranteed by a date and time or fully refundable.
Just a few heads up as we approach the busy season, if you plan on sending/receiving packages. Try to have a little patience also, with the way the internet has taken over sales...we are all delivering many more common "store items" so package volume has exploded. Even such things as paper towels and toilet paper are being shipped and delivered now through all couriers, along with all the normal things we deliver everyday.