In Pakistan, OCS is dealing this method, or you can visit yourself at HEC Islamabad, R&I Department Head will do this sealed envelope thing. As far from abroad is concerned, I am not sure, but, my long shot is, you have to courier to the same person that is dealing with it, and explain it with a cover letter, and include the self addressing courier bag as well (Like if you are sending via DHL / FedEx, pay them the return fee as well and include a bag), so that they can send you back. It usually takes 5 minutes to get them sealed in-person. Would probably take two - three working days.