a solo mailing with the big card using EDDM from USPS will cost you about 7k for 20k.
U can target certain postal routes for less money and higher per piece cost tho.
Friend of mine just calls local biz, asks a few questions, and sells them on the phone ...doing a co op large postcard....I use video ...his closing ratio is better than mine cause he goes out to pick up the check ...
https://www.youtube.com/watch?v=ZnZo2aoUMlc