Since I was using a permanent felt tip pen for the address write the address before inserting the card.
Set your sewing machine on long stitches and sew around it...I know, rocket science, right?
I sent my valentines is a similar envelope (using freezer paper)when my daughter got the card, she threw away the envelope; her husband picked it up out of the trash exclaiming; 'this is so cute, you can't throw this away.' yes you can. Sewing around it is easier than gluing and can be thrown away. I mean save the paper and glue it to some clothespins or something.