"So am I suppose to add a "Authentication for PayPal Express Checkout Behavior" somewhere as this is what seemed to take me to paypal? Now I am even more confused than before :("
the Authentication for PayPal Express Checkout Behavior is only needed for express checkout.
When you fill in the credit card info you are not using express checkout, you are using PayPal Direct payments.
Using PayPal direct payments, the customer will not be transferred over to the paypal site.
the error is pretty clear:
the ExistingCustomer column does not exist in the database
I dont see a reference to the ExistingCustomer column on the checkout or confirm pages, is this error occuring on the success page?
you say you are testing locally right?
Make sure that you are checking for the ExistingCustomer column on the local server database, it should also be on the remote database to.
IPN is only needed for paypal payments standard, yo u are not sing this payment method, so it is not necessary.