This is a great anecdote. The challenge as you scale and add sales folks is that more often than not, they will indeed promise initially requested extra features in an effort to close the deal and reach quota. Invariably this falls back onto product or customer support who look like aholes because the over promising sales rep forgot to mention adding the automatic breakfast delivery function isn't realistic.
Without additional context, the post starts out making you think the customer was offering $75K just to add a feature they wanted. They were actually buying the product anyway, but trying to leverage the sale as a way to add a new feature. Big difference once you realize this, and not such a difficult decision to say no to. The former would be.
Good advice, but the reality is, if you are running a bootstrapped company there will come time(s) when you have to take the money for custom work. It is all part of the hustle.