You can link your existing Salesforce payments-connected Stripe and PayPal accounts with Salesforce Order Management. For easy linking and integration, install the Salesforce Payments for Commerce Cloud package. For Stripe accounts, the package lets Order Management capture payments and issue refunds. For PayPal accounts, the package lets Order Management issue only refunds.
The package is available on AppExchange along with the installation instructions. Install the package on the same org where you installed Salesforce Order Management.