The page a payment link opens, and an invoice's card payment: Paddle adds the transaction to its address, and the page opens Paddle's checkout for it by itself. It says The checkout for your payment opens on this page. If it doesn't, reload the page, or ask us for a new link. Opened without a payment, it says This page opens a payment you were sent a link to. Use the link in your email or on your invoice. Once paid, the customer goes on to Purchase Complete.

Create a Pay menu item so the page has a proper address and your template, and hide it from menus if you like. Its address is also a good Default payment link in Paddle (see the section called “Setting Up the Checkout”), which Paddle uses for transactions created in its dashboard.