Using PaypalButtons - the online version.

gistfile1.js

document.addEventListener( 'DOMContentLoaded', function() {
    
    var parentNode = document.querySelector('#paypalButon');
    
    PAYPAL.apps.ButtonFactory.create( 'YOUR PAYPAL ID', {
        button: { value: 'buynow' },
        name: { value: 'Some Product' },
        quantity: { value: '1' },
        amount: { value: '5.99' },
        currency: { value: 'AUD' },
        shipping: { value: '0' },
        env: { value: 'sandbox' }, //keep it in the sandbox.
        callback: { value: 'http://staging.site.com/confirm/paypalIPN' }
    }, { type: 'button' }, parentNode);
    
});

Thanks Verikon!