I don't trust Paywithmoon, there are a few shady things behind it.

It collects your data without asking the users their permission by manipulating the DOM browser and you can not opt out. When you install the browser extension it knows what pages are open in your browser and much more.
When you initiate the payment using your Coinbase account, it extracts the API keys (without you notice it) and the keys are stored on a server they control in plain text, it's not encrypted.


Amazon doesn't care about cryptocurrencies and it prefers to focus on Amazon Pay


Microsoft, Overstock, etc accepted Bitcoin on their store. Did you see a "major" boost on the price?. I don't remember something noticeable People won't go to purchase Bitcoin (paying a fee) to be able to pay their items on Amazon (and paying a fee again). Why not just pay with regular methods (GC, card, bank transfer).
As someone mentioned, if there is a decent discount then it may be another story