But I’ve also received invoices rounded off like that.

Let's say I do a kitchen remodel and I get an itemized invoice from my contractor which breaks down the remodel into: $1500 for plumbing, $1000 for electrical, $2200 for cabinets, $1500 for granite and $1000 to remove a wall and drywall/paint. 

You provided an invoice and I OK'd the project and you disappeared. 

My property manager just called me/sent an invoice with 2 options regarding replacing an electric water heater (please see attached screenshot).

Enter a maintenance request, assign a vendor, attach a fake invoice.

I just got an invoice for 27000 for a bad tenant turnover today.

Either a General contractor bid or actual invoices.7.

There is significant $$$ to be made.To answer your question about retention I try to;1) Pay invoices same-day2) Don't beat them up on price3) Establish a face-to-face personal relationship

Of course I have a lot of details around this but an example is they seem to have "in house" contractors so they can't really furnish actual invoices and receipts for repairs.

In the event that any utility or service provided to the Leased Premises is not separately metered, Landlord shall pay the amount due and separately invoice Tenant for Tenant’s pro rata share of the charges.