Payment Setup

How do I set up the “Payment Gateways” in the “Payment Setup” ? What do I have to fill in the “Paypal email” field?
How do I link the payment to a Membership Package? According to the video example, the difference between the membership package and listing package is that in the case of membership packages the user has to pay before listing the products as opposed to the listing package option when the payment is done after the transaction is completed.