He said that when they spoke she asked him to rewrite the invoice to reword it in such a way that it would essentially state that there wasn't anything wrong with the sewer line. 

Hello,My renter just sent me an invoice for a reverse osmosis water system and installation fee.  

Or they do more work for more money, and this money is often gotten in various unscrupulous ways "under the table" from things like repair invoice markups.

Just because they don't list it doesn't mean they don't "charge it" by having their vendors mark up repair invoices and under the table kick backs of the diff.Buy local and self manage is your best route ... otherwise be prepared to go through several crappy, crooked property managers (spidey senses tingling on this one), hire and fire many, then maybe finding a decent one that you will still need to monitor carefully.

By the time you vet 16 guys, wait for them to show up over and over as scheduled, clean up after them, spend time licking and sticking checks to pay the invoices, figuring out if it's a deductible expense or not, on and on.  

Although I have no problem with a PM having his own company do the work, just gotta check up on the guy once in a while.And to be honest, we don't know yet if this PM is gouging or not, let's see some actual invoices.

Your concept of discounting is similar to a business invoice with terms like Net 0, less $100 5 days prior due date.I use popmoney.com to assist tenants in making timely payments and make past due expensive -- $150 late fee.I send a courtesy note the first time their rent is late:You are  late in your  rent payment and this memo is to remind you of the $150 late fee. 

More than half of this invoice  shows tree trimming & cutting.

Invoices for repairs go on the HUD-1 on the initial cash purchase right? 

This is provided on a monthly basis with each invoice that is issued to our clients.