Marketing to Multifamily Sellers!

Okay I have asked previously in regards to putting together my direct mail campaign to multifamily sellers, regarding the use of yellow letters and postcards. I was told to use a professional letter with a handwritten envelope and a stamp, since I am marketing to sellers of 5-20 units. I am a little confused because I have read other post which talks about basically the same thing but they were told to use a variety such as yellow letter, professional letter and postcard. I don't mind using all of them but which should go first, second and so on?