To setup a new payment gateway for use in Billing, you will want to navigate to Billing–>Gateways, and click “Setup Now” next to your desired new payment gateway.
Once a gateway is selected, the appropriate API and/or merchant fields will be displayed.
In the case of PayPal, you will be prompted to enter your API Username, API Password, and API Signature, along with a checkbox to turn “Test Mode” (equal to Sandbox mode) on or off. You can find information from PayPal regarding locating your account’s API information here: PayPal API Info
You can find information from Braintree regarding locating your account’s Merchant information here: Braintree Merchant Info
You can find information from Stripe regarding locating your account’s API information here: Stripe Merchant Info
Once all of your API information has been entered, just click “Save”. The new payment gateway will now be available to use by your clients when paying their assigned invoices.