If they move and change their phone number, you have nothing.I only accept partial payments from people who I know to be on the edge and I document it in the invoice and receipts.

For business, there's D&B, Experian Business and Equifax Business.Businesses can get "Net-30" lines with vendors (they ship product, they bill your business, your business is expected to pay the net invoice amount with 30 days).Businesses can get bigger unsecured lines than consumers once the business's credit is established.If you can buy in an entity, by all means.

And I have made sure I paid the last invoice - even though I disagree - when the property is still under my possession. 

Keep invoices/records for updates/fixes you do, and report one-time fixes on line 19 with an itemized "see statement 1" that matches the invoices.Feel free to write depreciation off.

I include the legal wording as required by the FL statute regarding security deposit deductions, and include my contractor's invoice.

Again, you paid for a service you received.Lets say you bought a refrigerator at the local appliance store, were making payments on it and then they went out of business but still sent a invoice for the payment.

If I don't get "paid", then of course they don't either.I was considering the "flat-fee" approach because I truly hate seeing all the nickel and dime charges for (paper, advertising, leases) etc..... however given the price difference between the (2), I think I can suck it up and look at each invoice knowing I'm still saving money.

Yes, Home Depot offers the line item invoicing on their Net 30 Commercial Card, which is nice, I use street address as my PO # to track expenses for certain properties.Not sure about Lowes, but I was just approved for their Commercial Card w/o a personal guarantee.

I invoice monthly, within a few days of the first of the month for the agreed upon % and any reimbursable expenses.Keep in mind AirBnB destroyed their Co Hosting feature by removing the ability to get paid through AirBnB. 

In Georgia - the state has always "invoiced" me annually for my LLC's.