Assuming the PayPal button is rendered by the PayPal JS SDK, similar to the demo at https://developer.paypal.com/demo/checkout/#/pattern/server , the only customization options are what the JS SDK provides for customization, as documented here -- for example, there are a few colors and a pill shaped option. Since the Shopify platform is rendering the button, only the options Shopify provides for changing its use of the JS SDK can be adjusted. I'm not aware of Shopify having any options at all for this, but that is what would theoretically possible. Other things within the PayPal button iframe (besides what's documented in the JS SDK reference) cannot possibly be changed.