Origins
Postage is the price of a network that no longer exists. The word comes from post — from Italian posta, meaning 'a station, a relay point' — from Latin positus, the past participle of pōnere, 'to place, to put'. A post was a point placed along a road.
The system worked by relay. A rider carrying letters would gallop to the next post station, swap his tired horse for a fresh one, and continue. The letter moved at the speed of a series of sprinting horses. Postage was the fee charged for using this network of placed stations.
The Latin verb pōnere is one of the most fertile in the language. Its past participle positus (contracted to -posit or -pose) appears in dozens of English words. Position: where something is placed. Deposit: placed down. Compose: placed together. Expose: placed outside. Impose: placed upon. Oppose: placed against. Propose: placed forward. Suppose: placed under (as a foundation for argument).
Later History
England's Royal Mail, established in 1516, began as a network of post roads with relay stations. The penny post of 1840 — Rowland Hill's reform that introduced the postage stamp — democratised the system. Before it, recipients paid on delivery. After it, senders paid in advance by affixing a stamp: proof of prepaid postage.
The word survives in the digital age mainly through 'postage stamp' and 'return postage'. Email, its replacement, carries no trace of the relay stations that gave post its name.