I haven't noticed the buy/purchase thing myself, and I don't use public transport enough to comment on the customer / passenger thing (though when talking about the passengers of planes and the like on TV they still seem to call them passengers mostly).
The way language changes is kinda twofold... one part of it is that new words are added to the lexicon, while some others pass out of use, for whatever reason. The other part of the process is that some words remain, spelling and pronunciation wise, but their meaning changes.
When St. Paul's cathedral was finished and dedicated 300 years ago, King Charles II inspected it and declared it "Awful, artificial and amusing". That is to say: Something to inspire awe, something made with artifice, and something to raise the spirits of men.
Chaucer's poetry from 600 years ago is "English" but good luck reading it.
The proliferation of printing presses after the renaissance, along with the consolidation of English and the compiling and publication of dictionaries in it in the 18th century, have heavily slowed the mutation of English - the US Constituation and Bill of Rights, written 236 years ago, still read in straightforward English today. Go back just another 100 years before that, and things are a lot more different.
The changing of language actually poses something of a problem for those who have to ensure the long-term safety of nuclear waste, some of it having half-lives of multiple tens of thousands of years. Suppose a societal collapse or cataclysm of some kind came along that destroyed modern technological civilisation and reduced mankind to a much less numerous, more primitive state - knowledge of these facilities might be entirely lost, and linguistic drift might cause people who came across them multiple generations after the cataclysm to be unable to read the warning notices.