A2Billing comes with PayPal integration built in so that A2Billing customers can add credit to their balance using PayPal automatically.
There are a few things to configure to get this integration working, which are listed below.
The exact setup will depend if you are running A2Billing 1.3 or A2Billing 1.4+. In version 1.3 all of the settings were in a file called /etc/asterisk/a2billing.conf. In versions 1.4 and later the settings were moved in to the database and can be changed via the web GUI. The examples here show the web GUI.