The only way to link your store or payment section to paypal is by linking the page to an external checkout page.
Please follow the next steps:
Please create an external checkout page on your paypal account (you can ask paypal support team for assistance).
2. Please go to your payment section on your site and add a new button above the store item called 'paypal' (please see the attached gif).
3. Link this button to your own external checkout page
(please see how it's done on the attached gif).
If you want to link the BUY NOW buttons themselves you'll have to remove the pricing from the payment item on your site and then perform the linking procedure.