Send them a past due invoice too. 

I live in Beaver, PA and the city sent a reminder of a bill that was invoiced and sent to the old owner on February, 2019.

Where can I send you the discounted invoice?

If you truly mean the best as the most expensive, invoices will be over $200,000.

I also wonder why lose a good tenant over only this and why was the plumber's invoice so high?   

Get the copy of his ID when he turns in the application (use your phone, lay the ID on the app to take the photo).Hopefully you are in a location where you can even evict - be sure you are adding his fees onto a monthly invoice, showing the running total past due.

When you receive the invoice from the plumber you can share it with the tenant for full transparency. 

Invoices reflect the current insurance pricing.

Tenant agrees to do so upon receipt of an invoice from LL.

The one exception would be if they own an official company that you would have considered anyway - in this rare instance I would still require a formal quote, invoice and pay them like any other professional.Never take “favors” from your tenants.