Voluntary Retirement is right of an employee with more than 20 years of paid service. That means he should have received 24 months salary without any break in service provided no disciplinary (major penalty) /criminal case is pending against him. He need not cite reasons or provide supporting documents and such request cannot be combined with any other grievance as it has to be unconditional and independent.
There is no need of Medical certificate of any sort. So I differ with Mr Kameshwar Rao.
Someone may exploit you and charge heavy expenses. Even the clerks in the Post office where he is working would have guided him. There is no special drafting needed. (Unless there are other material facts which you may be hiding) The draft is as under:-
Sr Supdt of Post Office
(through Post Master ________)
Sir, having joined service on _________ I have completed more than 20 years of qualifying service. I wish to seek voluntary retirement and hereby tender my notice of 3 months and would request to be voluntarily retired on __________.
If no rejection is received till last day of the expiry of three months then he need not attend duties from next day and claim his pension. Administration will pursue the matter as he is not the first postal employee making this request.