"Cheapest" is not in my criteria. When I share book links I always take the time to find the original publisher and / or author's website and pass that around. Pay an extra dollar, where it is also more likely that more of that flows to the author/publisher. It is very easy to do a book search, find Amazon or Goodreads in top results, nice cheap and quick delivery, and just share that around. And helping the utter dominance of Amazon to become even worse.