Actually I typed in the wrong number, my letter is also 20cm in length. If you look at the measuring ruler next to the letter, you'll see.
My box is also made out of thin but corrugated cardboard. However, you said the interior of yours is white, which does sound fishy. And left & right sides of your box seem a bit too flat.
Can't say for sure... but hopefully you didn't pay too much. If you need more pics to confirm the authenticity - assuming mine is genuine - let me know.