how to make sure what I mail gets there

I've had 2 situations where I mailed something that never got there and they were both important, one was some paperwork to my doctor to fill out so I could get para transit, I mailed it twice and she never got any of it, then when I messaged her online her assistant said to fax it, I found out they gave me the wrong fax number and they faxed it to the right one. I heard it takes a month to process so I'm waiting to get mail from them.
the other one was when I filled out something online from syr housing where I used to live and they called and sent me paperwork, I filled out everything and copied it the same day. this was back in may, I sent them a question online about it and wondered what happened that I never heard back from them, found out they never got the forms I mailed out. someone told me I needed to use their envelopes cause of the code on them, I did use theirs but ran out of room so I used mine too. why didn't the post office ever deliver them, had the right address, they're going to send me the forms to fill out again, how do I make sure they get them , I've thought of asking for their fax number. do libraries fax stuff? how can I send them in such a way that they get them?