I keep a new water heater in a box in one of my properties in case I need to replace one on a holiday or the plumbing supply place is out.Regular maintenance items are furnace filters, seasonal activations of basement/crawlspace heating, distributing salt buckets, and gutter cleaning. 

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Can I just write the letter and place it in their mail box or do I need to have it certified? 

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@Jason Collins I love how your thinking outside the box!

Including my 2 kids new bedroom suites that  were still in the boxes.

Some of my tenants pay by auto bank transfer; while a whole bunch of others deposit checks into a lock box at my apartment building.

Right now I'm using a PO box that everybody mails their rent to.

The only way to prevent that is to really screen your tenant to ensure they've been problem-free with previous Landlords while owning the cat and by conducting regular inspections to verify the tenant is cleaning the litter box regularly and preventing any odors.

Any light boxes sound puttied, edges caulked with acoustic caulking and then painted. 5/8 is best for fire separation and sound.