Here's another good integration of existing libraries into React Native. MattFoley has brought us PayPal through it's official SDK.
We really hope to see even more of them in the future.

react-native-paypal

Paypal wrapper for React Native, currently only supports the bare minimum of buying a product.

How To

You're going to need to integrate the official PayPal-iOS-SDK.
Simplest way to do it is through Cocoapods:

Install Cocoapods

sudo gem install cocoapods

# Move into project's folder
cd ReactPaypal

# Create a Podfile
touch Podfile

Paste this content into Podfile

workspace 'ReactPaypal'

platform :ios, '6.0'
pod 'PayPal-iOS-SDK'

Install and open

pod install

open ReactPaypal.xcworkspace/