Paypal has documentation on how to add PayPal buttons. Specifically, they say this on their HTML reference page:
Note: Buttons created using the Create a PayPal payment button page are secure and can be posted directly to your website without modifying the button’s HTML code. If your site configuration requires that you modify the generated HTML code, or if you need to create your own buttons manually, see Secure your HTML buttons to ensure your buttons are safe from malicious tampering.
Hugo should be able to handle this fine.