You cannot get delivery confirmation on "letters" or anything packaged similar to a letter, whether it is in a standard or manilla envelope. I went through this same thing 2 days ago.
If the envelope is thicker than a 1/4" it becomes a first class "package", which you can then get delivery confirmation.
I was told on my fabric postcards I could mail them in the manilla envelope for 44 cents, first class, no delivery confirmation, OR pay $4.95 for priority first class plus .70 cents for delivery confirmation... THAT was not going to happen for each postcard!!
I didn't find out until today I could have stuffed a wadded up clean kleenex in the envelope and mailed it for a total of $1.20 LOL
Some people will add one packing peanut to the envelope.