Everybody who works at the Postal Service can tell a real check from an advertising piece designed to look like a check. It is unlikely in the extreme that anybody would bother to look inside it, especially when you consider that it comes into the office in cardboard trays with thousands of identical pieces. Postal employees are practically hardwired to look at the address and then at the postage indicium, which in the case of your mail will state some version of Standard Mail or Presorted Standard. They know that the contents are valueless and when they open a piece of Standard Mail to inspect its contents for classification purposes they should stamp it "Opened for Postal Inspection".
Those letters are sorted on high energy transfer barcode readers. They are accelerated from zero to 15 feet per second in a minute fraction of a second by means of two high speed rubber-like belts. The corners and windows are very often torn in this process. Sometimes a jam will cause a trailing piece to impact the piece in front of it which almost always results in a small tear at the corner of the flap. Most people don't understand the stresses placed on mail pieces by modern handling and the minor damages that result, nor do they understand that most mail such as you describe is never touched by human hands until your letter carrier is walking up your front walk. This unfamiliarity with modern mail processing is the fatal mistake made by the Anthrax mailer who thought the dust was safe inside an envelope when in reality the envelope would undergo stresses he did not anticipate, causing the envelope to balloon, accordion, and open at the seams during the belt driven acceleration, spraying the stuff all over the equipment and into the atmosphere surrounding it.
Incidentally, I used to work on one of those machines and have personally processed tens of millions of pieces through them. I also supervised an automated processing operation with up to 60 employees who processed at least a million a day. This was in the facility in CT that became infected with the Anthrax that killed a woman who received a tainted letter there.
