Knit the envelope. Here's how.
Cast on 3sts. Row 1 - K. Row 2 - P.
Rows 3, 5, 7, 9, 11 and 13 - incr I st each end (you'll have 15 sts in all).
Rows 2, 4, 8, 10, 12, 14, 15, 16 - P.
Rows 17-30 - stocking stitch ie K one row, P next row.
Rows 31, 32 - P
Rows 33-44 - stocking stitch.
Cast off (bind off).
I put in those purl rows where you'd expect knit rows at rows 15 and 31 because it makes the envelope look crisper when you fold it.

Sew in the ends and then embroider on the stamp and address.

Now fold your piece of knitting into an envelope shape. Sew the side seams. Catch down the flap at the back.

SWALK if you like too!
(SWALK = seal - or in this case - stitch with a loving kiss)
This will be great for a Valentine gift!

Attach the key chain to one corner, either by sewing or with a jump ring. There you are - all done!