How can the VA send paperwork through regular mail that has a time limit on it for me to reply? Should this not be sent by registered mail?

I had applied for disability due to an injury to my knee while I was active duty. This injury is in my medical records. I had been fighting this for approximately 6 years. On one of the Notice Of Disagreements I was awaiting the decision letter to show up. I called the evidence intake center to find out the had sent it out already, which I never received, but was told my 90 days to reply was past and the claim was closed. No I can not reopen the claim because of no new evidence! My question is how can they send paperwork through regular mail that has a time limit on it for me to reply? Should this not be sent by registered mail? Because of this I was never notified because I never got the decision letter. Either by the Postal Service error or by the evidence intake center. How do I even know that they sent the decision letter out? During that time they were extremely busy and under manned. When all said and done this error has caused me much grief!!