This only applies to receiving payment in Accelo. If you receive payment via your online billing system then the receipt method should be automatically pulled in - though some systems do not provide this information, in which case the receipt will have no payment method set.
You can keep track of the various ways payment is being received in Accelo by recording the receipt method on a receipt.
As an administrator, you can modify the available receipt methods from the Admin page. Find Billing & Rates > Receipt Methods
Accelo comes with a basic set of receipt methods. To add more, click "Add Method" from the grey taskbar.
Click Save to complete adding a new receipt method. When receiving payment you will now be able to select this method.
You can also search for receipts based on the payment method: