Currently, Paypal, Plug'n Pay, Iridium and Moneybookers are the only online payment methods that A2Billing can use. Paypal is the most mature of the interfaces, and one could argue that it is one of the most trusted and well known. It is therefore recommended that Paypal be used over and above any other payment provider.
A2Billing can quote to supply interfaces to other payment gateways such as Worldpay.
Click Edit details to set up each payment provider, and there are some entries in system settings to edit under epayments. You will need to have the appropriate ports open on your firewall, in the case of Paypal, this will be port 445.