For using this addon on your paypal account should be enabled Reference transactions
As we understand now is not possible to enable it.
Install the add-on with commands:
apt install splynx-paypal-rb
After installation you have to configure
/var/www/splynx/addons/splynt-paypal-rb/config - folder. Look at the screenshot:
Settings are complete.
And now the customer can add his payment card to the portal page:
To do so he should login to his PayPal account and click on [ Agree & Confirm ].
That is all. Client's configuration is completed.
At last, you can charge all customers using one button! Go to Finance → Invoices, set period and click "Charge" as on the screenshot: