Adding a widget under an ‘Add to Cart’ button through a PHP snippet

Your plugin for the converter supports shortcodes, so this is a good starting point.
Everything should be as simple as using this code:

add_action( 'woocommerce_after_add_to_cart_button', 'astra_add_woocommerce_payments', 97 ); 
function astra_add_woocommerce_payments(){
  echo do_shortcode( '[currency_converter_calculator lg=”en” tz=”0″ fm=”EUR” to=”USD” st=”info” bg=”FFFFFF”][/currency_converter_calculator]' );
}

You probably want to wrap the shortcode result inside a div or something to size it properly for your add to cart area.

You can also take the “register sidebar” road and create a proper sidebar/widget area so you can control it from the backend, in that case take a look at this.