Go to the Magento Marketplace https://marketplace.magento.com/
Search for extension “PayApi”
Buy the PayApi extension for free
Login into the magento backend
Go to Settings > Web setup wizard > Component Manager section
Sign in with your Magento Marketplace account
** Access to https://marketplace.magento.com/customer/account/
** Once logged in, go to “My Access key”
** Create new Access key or/and copy the public/private keys generated previously
** Back to your store Admin backend and paste these keys.
Press sync button to synchronize with your store
Select payapi/* modules to install them.
Click “install” button and continue the installation steps.
Go to the Magento admin, open the menu option Stores > Configuration > Sales > Payment Methods
Open the PayApi extension section and type your PayApi PublicId and your PayApi API key (You can get your publicId and API key from here)
Select your default Shipping Method for the "Instant Buy" functionality (It might require to create a new shipping method if there are no methods)