The prepositional phrase in the sentence is "on the envelope".
It's "on the envelope" because it is showing place or time. Some key words are: -in -on -under -by -to
That is a rather thick envelope. The ship began to slowly disappear as the fog bank began to envelope it.
letter
The nuclear envelope surrounds the nucleus of a cell, consisting of two membranes that regulate the passage of materials in and out of the nucleus.
The attorney is carrying a large manila envelope.
The postage stamp on the envelope was colorful.
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'.
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.
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.