First class cannot have delivery confirmation, a first class parcel can though. You cannot just get tracking on first class. For it to be a first class parcel, it has to meet certain criteria.
Yeah and it doesn't take much to deem it a "parcel". I send 1st class all the time with delivery confirmation. The criteria is the thickness of the envelope. That is why "padding" the envelope was mentioned by cyberdan.
These are the specifics as to what you
cannot have delivery confirmation on-
Letters-
Must be rectangular.Minimum size is 5" long x 3 1/2" high x 0.007” thick.
Maximum size of 11 1/2" long x 6 1/8" high x 1/4” thick.
Maximum weight is 3.5 oz.
A letter will be charged a nonmachinable surcharge if it’s a square letter 5" x 5" or larger, it doesn’t bend easily, has clasps or similar closure devices, has an address parallel to the shorter dimension of the letter, is lumpy, or the length divided by height is less than 1.3 or more than 2.5.
Large Envelopes-
Must be rectangular.
Minimum size is either more than 11 1/2″ long x 6 1/8″ high OR 1/4″ thick.
Maximum size is 15″ long x 12" high x 3/4” thick.
Maximum weight of 13 oz.
Large envelopes that exceed these dimensions will be charged package rates.
All you ever wanted to know (and then some) about USPS 1st class mail-
https://www.usps.com/ship/first-class.htm