Tenant says he mailed rent

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Tried searching the forums, but nothing quite fit my scenario...

I inherited a tenant when I bought an investment property in May of this year. He has paid rent almost always on time and without too much of an issue. He is blind and dependent on his daughter and nephew to help with the money order and mailing, so I've overlooked him being a day or two late when he couldn't get to the post office or he lost my address somewhere on his computer.

He insists he mailed the Nov rent on 11/1 but has no receipt for the money order. He says he did not detach the receipt prior to mailing...he has done this in the past, so it's entirely feasible he did this again. His rent is due on the 4th so he's now into late fee territory and I'm trying to figure out what I need to do to protect myself. I'm inclined to believe him when he says he mailed the rent (or at least believes he mailed the rent), and based on that I would prefer to keep him as a tenant, but there's no way he could cough another rent payment. Bottom line, I'd prefer to keep him if this is indeed an isolated incident, but I need my rent money.

Other notes: He is still answering my calls, so he's not avoiding me as of yet. He says he has an appointment with a counselor a week from today to see if they can provide money to help him with rent.

I was planning to propose that he pay an additional $50 per month until the $700 is paid or until he moves out and I can recoup the balance owed from the deposit. Any comments on this plan or other ideas?