You need to use a third party application to buy amazon products using bitcoin called, purse.io! However, I have not used their services personally but heard about them. But I will suggest you to read the below link before you begin using Purse services,

https://www.reddit.com/r/Bitcoin/comments/3f36s5/before_you_use_purseio_a_few_things_everyone/

Secondly, there are lot of ways to buy bitcoins. I don't know about Robinhood but you can use Coinbase for your crypto purchasing need. Maybe someone else can help you for robinhood. But usually a little Google search will reveal more options specific to your country!