@Max H.
What to send has more to do with what your business model is then much else.
Yellow letters give back the most calls which doesn't mean they give back the most contracts... They may or may not.
This is going to sound strange coming from a guy who sells millions of yellow letters however most trainers push yellow letters not for the contract success rate but for the call volume success. A blast of busy work from a 1000 yellow letters tends to make trainers look good. Where in reality it may not be the best course of action.
I would much rather someone starting off understand that a majority of the calls will be non motivated leads. That said if you have a system of follow-up then building relationships with the non motivated callers will produce ripe fruit down the line.
The issue with that is that most investors starting off need ripe fruit right out of the gate not 6 months later.
Postcards are going to give off far less busy work and tend to have a better motivated caller. However it can be a little frustrating because of the lack of volume. The volume issue can be rectified by sending enormous amounts of postcards.
In reality most trainers who are active send Postcards over Yellow letters however they do them in such large numbers that the result fit their expectation.
The zip letter sits in the middle of the two in that its open rate is huge thus pushing off a higher call volume than postcards and the caller tends to be at a higher level of motivation. Reason being is that we can say more and offer more opportunities for them to contact us.
Falling back to postcards; Size matters. the small cards are advantageous only due to cost. 36 cents to get a piece of mail out is great however the larger cards pull more then twice the call volume and are only a few cents more per piece.
I've begun using a 6x9 full color one side with text on the other side and love the results.
Then to add to the "What to send syndrome" we are also using greeting cards specifically designed for investors who want a great way to build relationships. A real mom and pop approach.
Hope I helped some with your decision of what to do and how each affect you.
And thank you..