From what I understand, the transcripts are to be sent directly by the university, but the degree certificate is to be sent by you. They are different documents to prove different things. I had to do the same.
My university sent my transcripts directly to WES. I requested them to put my reference number on the envelope which they did. Then I sent a copy of my diploma and put the reference number on the envelope as well. They were able to connect the two and evaluate.
Not necessarily. I sent both mysef in a sealed envelope stamped by my uni. Every country has its own requirements.