Some usability things I noticed on the Drop-in UI Sample Page: (https://www.braintreepayments.com/features/drop-in)

It appears that the PayPal is a header, rather than a button, it has no call to action. Separating the credit card form from the button and giving it a clear header (Like: "Pay With Credit Card") would help indicate the choice of PayPal or Credit Card.

Also, I'm not sure if it's intentional, but the credit card form doesn't actually do anything. Completing the form with a test credit card gives no follow-up call to action. This could be confusing to people wanting to see a "working" integration.