.- I mailed out my postcards at a quantity of 300-400 and an interval of once a week.

I'll not bore you with the details, but I will say that the tests I'm running are statistically significant; +-5% interval with 99% confidence.I hate to harp on the math, but it is quite important to this conversation. 

Balloon payment due in 36 months.That means that in 36 months you have to redo the loan(continue at same payment rate with a possible interest rate adjustment) for the remaining principal or pay it off(the remaining principal)This gives the bank an out at scheduled intervals.

You can send in intervals of 3-4 weeks between batches.

I found it best to stick to printing in 100 page intervals, not sure why but that seemed to help.

I will put 2-3k miles on this car while I own in and I don't want it to be near a maintenance interval when I'm trying to sell it. 

Do you find that the one month interval is the best?

The best advice @Carlo Palomino is to "test"...Do a little research here - there were some great responses above...and then decide which Strategy seems the best one...and then test...If you want to improve your test results - Split test...Divide List and try 2 different intervals to see which one produces the most number of deals...In real estate its all about "Location, Location, Location" in Direct Response Marketing...It's "Test, Test, Test"...What you will find real quick when asking around is there is No "Magic Number"...

It would be nice to have a reminder system to alert me to send a new batch of 2nd or 3rd letters at specific intervals.

They typically only leave in there for 3,6 months to 1 year at a time in intervals so that the money stays liquid when they need it.2% returns do not cut it long term as you are not even keeping up with inflation and then getting taxes on your so called laughable gains.In NNN the returns can be much better than that but it does take a lot of money.