As of March 29, 2017, redirecting to the old PayPal checkout experience will no longer process the transaction, and customers will receive an error messages on their payments. PayPal has updated to a new, streamlined checkout experience, and customers and integrations must be compliant. Is the NationBuilder integration compliant by PayPal's streamlined checkout process?

Here's the link for reference: https://www.paypal-knowledge.com/infocenter/index?page=content&id=FAQ2067&expand=true&locale=en_US

Old Checkout	New Checkout