The Google Analytics referral exclusion list is used to move domains so that incoming traffic is treated as direct traffic rather than referral by Google Analytics. When you add a domain to the referral exclusion list, people who arrive to your website from the excluded domain do not trigger a new GA session.
Checking your exclusion list
Navigate to your Google Analytic site
Click on Admin -> View -> View Settings -> Exclude URL Query Parameters
Sites such as PayPal, Klarna and Stripe will redirect off site for their tool and back to checkout.
However, when you add PayPal.com to the referral exclusion list then all the sales that were earlier attributed to PayPal is then attributed to direct traffic.
So, there is absolutely no advantage of adding these sites to the referral exclusion list.
Adding these to the referral exclusion list WILL NOT make the source revert to the original source of sales.
The best way to see the original source of sales is to use the payment gateway so that website users can complete the purchase without leaving the site.