The more personal and handwritten your letter looks the more attention you get - a plain advertising postcard usually does not prompt that many angry calls, but is also less effective.

Have you ever sent out hand written post cards or just letters?

I graduated with a computer science degree and found a way to use the county website photos and address to send out postcards.

When sending direct mail what gives you a better response rate, is it a handwritten letter, a post card with a picture of the property, what kind of direct mail has given you a good response rate?

If you send only handwritten letters or only typed or only postcards youre doing yourself a disservice. 

If I mail this list again I'm going to go with postcards for a couple rounds. 

Then went through gobigprinting.com for the post cards which I had a good call back return.

Greetings, I'm looking for recommendations for direct mailing service company for my next batch of "we buy houses" postcards

That was a waste of 800 post cards plus some angry dude in Texas. 

The edits came as I moved along.I would focus on the basic skills you'll need to make your cold calls, read a few yellow letter/postcard templates and come up with your own... then get started.