I'd give them written notice (which can be done with text or email, but I'd also send a hard copy with proof of delivery to whatever address you have for them) that because they have breached your written agreement by not paying you as agreed, the lease agreement is void. 

Vacuum and sweep the floors, outside steps and walkway.Weed the garden bed.Before attempting to clean stains from carpets and other surfaces, please call us so we can recommend suitable cleaning methods.Do not attempt to clean carpets with a shampoo machine as doing so may cause damage to the floor covering and the floor beneath.Utilities.You are responsible to pay for utilities through the last day of your rental period.Call the utility companies in advance and let them know the date of your last day of tenancy.Be sure to change mail delivery and subscription services to your new address.Inspection.Please let us know when you have vacated the unit and make an appointment with us for a final walk-through.You are not required to be present for the move-out inspection, but we highly encourage it.Before we begin the inspection, we will ask you about any known damages that occurred during your tenancy.Please note, it is often less expensive for us to repair damages than for tenants to repair damages, so talk to us first before attempting to repair any damage.The unit should be thoroughly clean both inside and outside and be ready for the next tenant.Final Report on Deposits.Within fourteen (14) days of our regaining possession of the unit, we will mail to you a complete written accounting of any money withheld from your security deposit and return the remainder.We will deduct for extra cleaning, damages, unpaid fees, unpaid utilities and unpaid rent, if such apply to your situation.Please provide us with your new address.Thank you.

If you knew about it, and are sure you owe it, you should pay it with proof of payment/delivery.

And I'd post it on their door or hand it to them, and I'd also send it to them via USPS mail with delivery confirmation.You can print out your own priority mail with delivery confirmation online at usps.com. 

You just send it with delivery confirmation to their last known address, which would be your property.  

Be sure and send it with delivery confirmation, so you have proof they received it.

The unfortunate inconvenience is that since the bottom doorbell is lit up, visitors or delivery people for the 1st floor unit assume that is their doorbell (when it's actually mine).

I guess we just wore them out so this is my last word on the subject.The quote was from the Florida decision so the Court, not me, was looking to the other states for direction, not precedent.I wrote that generally that's the definition of marketability, the Definitions section of the Conditions of the 2006 ALTA Florida modified Owner's Title policy provides:(k) "Unmarketable Title": Title affected by an alleged or apparent (emphasis added) matter that would permit a prospective purchaser or lessee of the Title or lender on the Title to be released from the obligation to purchase, lease, or lend if there is a contractual condition requiring the delivery of marketable title.I believe Nat's title meets that definition.I agree there's no cloud on her title because I believe she may not have title to have a cloud on if her units were not legally created. 

This not only adds to the project cost but also a long delay in delivery time.

Be aware if your jurisdiction has “1 business day” delivery times or similar for digital service you may need to adjust scheduled dates