In addition to what's been said above, the invoice should clearly state the address where the work was done and your company's name.

We were at a loss for how to get cash to purchase additional property, and are considering taking out a mortgage on the duplex.

If there is a market crash, don't kid yourself, it will be felt in real estate.

These kids already spent their house money on their education.

@Nadir M.Two additional thoughts:1) A central boiler is a cost trap for owners.

In addition you'll have to make the monthly interest only payments.

There is additional reserve requirements when buying more than 1 unit, so get up to speed on those so you are prepared to have those in place at the time of closing your loan. 

These are professionals with additional training and a stricter code of ethics.

Some random kids from the nearest high school?

basically Houston doesn't believe in general licenses. how to hire onehttps://www.biggerpockets.com/renewsblog/2015/03/1...as stated above try to find a bonded/insured contractor.managing your contractorshttps://www.biggerpockets.com/renewsblog/2010/04/1...additional tiphttps://www.biggerpockets.com/renewsblog/2015/05/1...paperwork to help make sure you don't get screwedhttps://www.biggerpockets.com/renewsblog/2010/04/0...Thoughts?