This Shomer-Tec Mossad Pen looks like an ordinary writing utensil, but it's quite the stealthy trickster. After you've penned your secret missive, run a hot blow dryer over the paper or place it near any heat source, and that black ink disappears.
Once the message gets through enemy lines to your recipient, all he has to do is hold the paper in the freezer or close to anything cold, and the message magically re-appears.
Write on the back of already-printed sheets of paper for even more privacy, but don't press down too hard! You can change the ink from visible to invisible as many times as you want. Spend $28 on this clever plume, or save your money and stop being so secretive.