The sheet of paper upon which Marcia writes the letter is badly crumpled (having been retrieved from the wastebasket), and she folds it in four to insert into the envelope, half in each direction. At the newspaper office, the same letter is being handled by the publisher on a sheet of paper that shows no signs of being crumpled, and that is folded in three accordion style.
When Marcia wants Mike's help with homework, Carol offers to help her, and Marcia responds, 'Mother it's math,' implying Carol is bad at math. However earlier in the season, Alice offers to help Greg with his math, but he wants Carol to help him because she's so good at math.
When Marcia climbs out of the window to mail the letter the bedroom light is still on. When Mike and Carol go to check on the kids the light is out but Jan and Cindy are still sleeping. No one would have shut off the light or Jan or Cindy would have noticed that Marcia was missing.