In terms of collecting, stamps are nonfungible. They are individually graded for the centering of the image, the condition of the perforations, the condition of the gum, and sometimes the novelty of the cancellation (if it's used). The price of a stamp is based both on the rarity of the stamp in general and on the grading of the particular stamp. When you buy a stamp, you're buying the exact pictured stamp[0], not a fungible equivalent.

[0] Usually. Sometimes people will buy "spacefillers" where they don't care about the quality, they just want something cheap to put in the spot in their collection. These are often unpictured and you get whatever the dealer sends you. These could be considered fungible.

Stamps had a function, now they are mostly decorative.

You can print postage with unique number. Stamps are just one method for postal service to identify your package and its payment and transportation mode.