To all who have questions about sealed attestation letter/certified copies of the documents.
Guys, do not complicate your own life. Just do what the application guide asks you to do. The attestation letter does not need to be in a sealed envelope---it is not your transcript. And certify the photocopies only when you're asked to, for example, for the translated documents.
I submitted the attestation letter my Graduate Studies office had prepared, and did not certify the photocopies of my documents, and was just fine to get my PER. (Except I had to appeal the decision due to the adaptability points, but that's a whole different story.)