It is fine the way it is. If you think the non-restrictive clause leaves too much room for interpretation (sometimes it's better to be extra explicit about what is and is not negotiable in contracts!) I would recommend some other changes to preserve clarity. If you simply omit the comma, the unbroken phrase becomes too long and confusing. Try this:
A certified letter setting forth the date the property will be returned to the lessor should be sent to the lessee as an advance notice, provided that particular date does not fall within 45 days after such a letter is delivered.
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