I want to invoice a landlord for property management services. On my invoice when I add a new product or service, should it be an item or a flat rate?

I can help you with creating an invoice for your property management services, @leemastergt-gmai.

You can choose By item if you're charging a product you sold or service you performed or a Flat rate if it's a fixed amount. You can choose either of the two based on what will work best for you. 

In case you need to track sales tax, the following article will help you track it: Manually track sales tax in QuickBooks Self-Employed. 

Let me know if you still have questions in creating an invoice. I'm always here to help. Have a great rest of the day!