Very cool.
The German Post already has "digital stamps", you buy them online and then you just have to write a couple of symbols on the envelope.
If you combined these together you would get a letter which can be fully paid for and addressed by writing a couple of symbols on it, only meaningful to the computer system. In a way this is making the physical world quite digital.
You can buy digital stamps for the USPS as well. You need to accurately put a barcode on the envelope though.
The point of the digital Stamps is that you just need a pen and write a couple of letters and symbols.