Cart and Paypal integration

I recommend Omnipay. It’s made by the PHP League and basically let’s you learn one payment framework and then integrate with almost any payment gateway (PayPal included): https://github.com/thephpleague/omnipay-paypal/

On PayPal specifically I see they have PHP SDKs posted on their developer site so integration should be easy. But I agree with jreklund, I don’t think such a thing will ever have a place in the core framework.