Even if you send out one letter a day, that is five a week, twenty per month. 

Doesn't sound like it will be a particularly hard model to copy, and when the word gets out you could find 10 houses on the same block under cutting your prices by 50% or more. 

Having a Pre-approval letter or better yet a loan commitment and a Banker to call and reassure the sellers agent you can close in 30 days or less also offers a huge advantage.In this market using a DPA is not a bad idea since rates are high and when they drop you simply refinance for a lower rate and payment. 

But 1) I bet you $100 every one of their neighbors has their ac set below 80. 2) when it’s 115 degrees out I guarantee everyone on my block has their ac set below 80, pry below 75. 

Direct mail has a very low response rate and I mostly just see the big boys who are going 7 figures do it, who don't mind spending 10-20k+ month on blowing out towns of letters/postcards.

Our rental is paid for and has rental income of $2200 month and our primary has a mortgage of $1800 so we are currently living for free basically.we have a credit line on our rental for $275k and recently got a letter from our mortgage company offering a cash out refi on our primary of approx $300k and still be able to keep our current interest rate on the remaining balance of the mortgage which is mid 3 %.We are trying to grow our rentals and struggling to figure out the best way to do that.

If nothing else it's an education and starting block for your journey. 

You may get a couple more letters, one from the old servicer and one from the new servicer, if that has changed.You are the trustor, so nothing changes for you.

Two dates after closing a letter arrived addressed to the previous owner from a lender about late payment on loan.what steps to take?

If they use your credit and income docs to formulate an offer, you can ask for a pre-approval letter (if you want), or just analyze the pricing and ask for a pre-approval from the top 2 lenders.