By default no payment processors are active. After selecting the Payment Settings tab you will find that there are two types of Payment Processors: Offsite Processors and Credit Card Processors.
Offsite Processor – Essentially a payment processed outside of your site. These payments can include external payment providers like Paypal 1 and Check or P.O. payments. Virtually an unlimited amount of offsite processors can be activated.
Credit Card Processor – These are the credit card payment processors. You’ll notice that only one credit card processor can be activated at a time, this is by design since there’s no viable reason to accept CCs from multiple processors.
If you ware accepting credit card information on your site you will want to use SSL for your site to keep your client’s data secure. Having SSL on your site is highly recommended for more reasons than accepting CC information, since every WordPress site has at least a login form.
After selecting the processors you want to accept for invoice payments and saving the processor options will be shown. Each payment process has its own settings, review and complete each option before saving again.