I really hate to get into this, but I do have to say that most of the Post Office employees are just like the rest of us, poor shlubs who are trying to do their jobs and make a decent living. I have gotten better service in the last few years from USPS than I have from UPS or FedEx, from accuracy of delivery date to delivery. Yes, there are a few bad apples, and they tarnish the reputation of the whole system, but I do not think they are in the majority.
To the OP - keep after the Post Office. File any and all claims that you can. Keep after ITC to file claims. Yes, sometimes items do get lost in the mail, but, given the volume that the Post Office deals with on a daily basis, these incidents are in the minority. Keep pushing for answers, but be polite. I am 65, and have been dealing with customer service issues for a very long time. I have found that if you can inject some personal element - "I know that you guys have been backed up due to the weather", or whatever, they are more likely to take your call personally. This approach has worked very well for me over the years. Keep things polite and businesslike, but let them know that you are upset and ask them for help to remedy the situation.
Around a year and a half ago, my daughter had finished her externship and was looking for a permanent job. Money was very tight. I ordered some eliquid, since we were just about out. The eliquid shipped, and tracking showed that it was out for delivery. No vapemail when the mail was delivered. I called the Post Office, in a panic, since this was the day before July 4, and there would be no mail for the holiday. I was informed that our street is divided between 2 routes, and the package must have gotten into the wrong bin. I think I really sounded panicked. I was trying to find a vape shop close by where we could get something, anything, in the emergency when I saw a Post Office vehicle pull up outside the house. They must have gotten hold of the other driver, who walked up the driveway and personally handed me our vapemail.
I can't really complain about the Post Office service I have received over my 65 years on this planet.