As others have also noted, any existing coin that gets accepted by Amazon will have to handle a massive increase in transaction volume. If it chokes on that increase, investors will judge the entire crypto sector quite harshly. That will be a great time to buy more coin.
As for Amazon creating its own coin, what is the use case that would both benefit Amazon's operations and make the coin worth buying?
Excellent point! If they did create their own coin, what would be the asset behind it? It would have to be used as a "vessel" for investing. And in that form the value would be what the traders make it to be? So, then they may have to resort in using a variety of cryptocurrencies (coins and tokens) in order to handle the massive volume. Something with a high volume. However, that is assuming they would "only" accept crypto as a form of payment which would never happen. So a combination of accepting both fiat and crypto as a form of payment could work.