Taking into account the impossibility to change something in the payment system settings on the site, the settings boil down to the following algorithm:

Go to the payment acceptance settings and add PayPal, set the billing address - skladusa.pp@gmail.com and save the changes.

Then open in another browser tab for adding a store in the SkladUSA system

(https://system.skladusa.com/web/ru/admin/user-shops/create) and configure store parameters:

Click "Create".