大家遇到过学位证也要放到骑缝盖章的信封的学校吗？

最近就要入学了，由于申请的时候邮寄的正式成绩单都没出问题，所以这次入学之后邮寄的成绩单和学位证就没在意。结果前几天问了下学校说不符合规定。其中有一个成绩单的问题确实是我自己理解错了，但看学校的意思好像是学位证明也要放到信封并骑缝处盖章，邮件原文：

We did received documents from SCHOOLA and SCHOOLB through snail mail on July 6th of 2016. While the transcript from SCHOOLB was considered as Official, the transcript and degree certificates from SCHOOLA and the degree certificates from SCHOOLB were considered as unofficial. This is due to the fact that they were not received in sealed envelopes from the respective Higher Education Institutions but sent together in one envelope sent by you.

请问大家遇到过类似情况吗？学位证明不都是学校开张纸里面盖个章，不放信封里面直接邮的吗？我今天去问问学校，但不知道学校能不能提供信封和盖章服务。。。

另外弱问引文里的“Higher Education Institutions”是指学校而不是某个特定机构吧？？