Add ability to display custom text on the order confirmation page

Currently, there is no way to add custom text or messages to the payment confirmation pages (success or failure) shown in the browser. A simple option to display additional lines or a toast notification on these pages would be helpful — for example, to inform customers to "Check your email for download link and invoice copy" on success, or "Payment failed, please try again" on failure. This would help reduce post-purchase confusion without requiring any major UI changes.