I'm learning that what Michael Quarles is talking about is true.
Recently, I've been looking into creating yellow letters myself.
First, I created a font with my own handwriting.
Then, I ordered invitation style envelopes that I will print myself using the handwritten font.
I ordered some multi-colored filler paper, college ruled, just like notebook paper you use at school. I'll print my letter on that using my handwritten font.
After that I will fold, stuff, and insert a business card magnet.
Finally, I'll take it to a presort mailing service in town who will print the postage at about .18 a stamp!
There is more work involved this way but it saves thousands of dollars per mailing for me.
The cheapest yellow letter company I've found charges .78 per piece. With 6000 pieces I spend $4680.
Doing it myself, including magnets, costs
about $3,000; without magnets it's about $2,000. (that is .33 per letter)
Hopefully all goes well printing that many letters on my home computer using 16 lb, 3-hole punched paper!
Eventually I'll get a worthy printer, and a pitney bowes meter to do my own bulk mailings. Then maybe i'll be a yellow letter vendor myself!