Frame it as a service to them - ask if anything needs repair/rehab. 

Does anyone know what typical water testing costs would be?

It's more likely that people reading this would decide against using your services.  

@Walter Correia, You're going to see a range based on service level, economies of scale and geography. 

That all depends on if the rent will fully cover mortgage, tax, insurance, 5% of rental income for repairs, another 5% for CAPEX, another 5% for vacancies, and another 10% for PM services (even if you self-manage).

One thing that I always considered with Cozy, which never arose in my rentals, was that a tenant could pay by credit card, pay the service fee and it would still be less then the late fee.Best of luck- I am a Zelle landlord now.

One of the reasons to enlist the services of a property manager is to insulate you from the tenants.

We also have a discounted repairs service, insurance and loan programs. 

Due to continual issues with my current servicing company, I may actually lose my home.

Once capitalized, the cost can be recovered over time through depreciation after the property is placed in service as a rental.