Using Paypal's Add To Cart API, may seem like an easy solution now, but I would encourage you not to use that option. You should only use the Web Chechout API. Then you should write your own cart or from packagist.

By managing the cart yourself provides the opportunity to save carts to a profile, provide upsells in cart, or use other payment processors (Stripe, Amazon, Authorize.net, Google). That way, if you ever become frustrated by Paypal for some reason, you can simply disable that payment method, and use another.

As for the Paypal Web Checkout API, you basically POST form variables to them with your invoice details (name, address, amounts). Very simple to use, and should require any special library/package to handle.