Pillows
Description: A reminder for Bitcoin insiders that there will only ever exist 21 million bitcoins, i.e. 2 100 000 000 000 000 (2.1P) Satoshi, with the word Satoshi in Japanese characters (in Japan-red). I also have a design with the absolutely exact number, but 21 with a lot of zeros looks nicer, and more people may recognize it, and it's pretty darn close to the exact number...
Description: 2.1E15 = 21,000,000 times 100,000,000, i.e. the maximum number of Satoshis that will ever exist. Sure, it's rounded, but close enough. This one is just the number in scientific notation, without the word Satoshi, to make it even more cryptic. Recognisable for Bitcoin insiders only.
2.1E15 Pillow
$28
Description: The formula to calculate the maximum amount of bitcoins that will ever exist. The block reward starts at 50 and is cut in half every 210,000 blocks (roughly 4 years), 32 times. In orange of course, with a half transparent black contour to make it readable on products of all colors.