There are a few different plugins that you can use to add Amazon Payments to WordPress. One option is the Amazon Pay for WooCommerce plugin, which you can get here: https://woocommerce.com/products/amazon-payments/.
Another option is the Amazon Simple Pay plugin, which you can get here: https://wordpress.org/plugins/amazon-simple-pay/.
Once you have installed and activated your chosen plugin, you will need to follow the instructions to connect your WordPress site to your Amazon Payments account. After that, you will be able to start using Amazon Payments on your WordPress site.