I opened the envelope that contained my check.
the cells have a nuclear envelope.
The prepositional phrase in the sentence is "on the envelope".
That is a rather thick envelope. The ship began to slowly disappear as the fog bank began to envelope it.
letter
The attorney is carrying a large manila envelope.
The postage stamp on the envelope was colorful.
I can give you several sentences.I mailed the envelope with the check inside.The earth is surrounded by an envelope of air.We need a larger envelope for this report.
The word its is a possessive form of the pronoun 'it', used as an attributive adjective.It is less awkward than saying 'Put the letter in the letter's envelope' or '... the envelope of the letter'.
Sealing usually means, to be encasing something in an envelope or another object, so a good sentence would be:When sealing an envelope, make sure that it is pressed down firmly.
When you mail letters, write the recipient in the middle of the envelope.
I went down to the post office to deliver my envolope.
My private letter to a loved one was sealed in an envelope and posted.