You want a higher rate of return a personally addressed letter is more effective than a post card. When we did the intensely time consuming letters we saw rates of return as high as 7% and as whole we signed up lawns at a rate greater then 1%. By the end of our second spring we had sent out probably 7500 letters and had close to 130 accounts from nothing 2 years prior.
The post cards had significantly reduced rates seldom better then 1.5% rate of return and as the years passed and the competition became more intense and the internet more a factor those rates tanked bellow .5%. But they are easier to make and send out and they were cheaper so 6000-8000 a year was normal. And we didn't burn out a 500-700 dollar printer every 24 months.